Of course in a world where even magic is common, still most of the population don't possess it and are regular folks. But the problem is that the ruling government/king etc can hire powerful wizards, clerics and paladins to "cleanse" the monsters, cursed forest, undead, werewolves, curses etc (the classic creatures and curses in gothic themes are often weaker compared to high level characters). And I mean what is so special in curses, werewolves, and other classic things when magic is common? They lose their scary element in that.

Maybe gothic theme is more fitting into a low magic world where magic is still mysterious and taboo or even forbidden like witch hunting?

The ruling powers can, but will they? Regular folks don't have the funds that the elite do, so why would the elite do anything about it until the problem is big enough to affect them? And if they do hear about it, how high up their priority list is it, compared to what they already have their magic users doing? If they're just hearing sketchy rumors about people getting lost in a forest, possibly even that it's cursed, are they going to take their mages off a battlefield, or are they going to send a footsoldier or acolyte to check it out? IE, a perfect backstory for one of your players.

In addition to what pocketmouse wrote, another thing to consider is that those powers that he might actually be in league with the gothic horrors. Vampires, curses, werewolves… those all become much scarier when the Duke is in on it. If the nobility are under a curse that makes them deranged, or are vampires or werewolves that’s a whole different matter than just some random monsters.

@pocketmouse @IamSposta thing is, actually the motive can't be just financial,so some powerful wizard can still choose to fight without money, as morals or such. In addition, some aspects of the classic horror theme lose their mystery and horror when I have high amount of magic, like a cemetery or a cursed forest. What is a cursed forest to a world who is known to magic? Also other actions can lose their mystery, like a scroll from a different language, I can just give it to a mage to translate. Or a shamanic summoning portal, I can just bring another mage who specialises in portals.

A few points to remember:

• If high-powered magic exists, that means the evil ones can use it too.
• The BBEs should always, always have more power at their disposal than the heroes or there’s little to no challenge.
• Spells like comprehend languages specifically cannot decipher codes.
• Finding the mage who specializes in portals is not going to be easy, and convincing them to help even harder.
• Just because powerful magic exists doesn’t mean magic is commonplace. It’s not like 17th level Wizards hang out on every street corner, they can exist but they’re still rare. Hells, any caster above 4th level should still be hard to come by.

You seem to have your mind made up about this already, but, like, Ravenloft is right there

Gothic horror isn't about particular monsters or settings -- at its core, it's about the past haunting the present. That can mean ghosts, it can mean generational curses, it can mean simply memories and trauma and nothing supernatural at all, like Hitchcock's Rebecca. Those themes can resonate in a high-magic world just as easily as a low-magic one

From Van Richten's Guide to Ravenloft, quote,

• Gothic stories include intense, even exaggerated, emotions. Romances, rivalries, and life-changing events are common in these adventures.
• Atmosphere and a sense of dread are key to achieving a gothic feel. Set your story in an decrepit mansion, ruined cathedral, or other foreboding location.
• Gothic heroes are often virtuous, deeply passionate, or courageous. Find ways for adventures to test characters’ beliefs and morality.
• Gothic villains are unrepentantly evil, but this shouldn’t be immediately obvious. Drop hints about your villain’s awful secrets before revealing them fully.
• Sacrifices feature prominently in gothic stories. Give characters heart-wrenching choices to make.

None of that really says to me that high magic can solve those problems. Hell, it caused quite a lot of them in Curse of Strahd, such as the good Count's immortality and getting himself wound up in the Demiplane of Dread. You don't get much more high magic than trapping yourself in an angst prison.

Gothic Horror lends itself very well to creative problem solving. Breaking curses and saving lives is going to involve a lot more than Fireball. Cosmic horror in particular makes a point of how it's so much bigger than us, that the fact we're thinking of what weapons will defeat the enemy proves we don't understand what we're up against. You can't shoot Cthulhu any more than you can magic away a ghost in this genre.

My final point to make is to have players use their brains, not brawn. High magic isn't exactly working out for the NPCs in your Gothic setting because it's still a Gothic setting and not a magical paradise. Players don't need to be more intelligent than the wizards of your world, just smarter.

If you're really concerned with "high magic" trampling on "classic gothic", you might have to level up your gothic big bads to things like Mind Flayers, Aboleths, and other abominations from "beyond...."

Other thought, maybe magic is compromised so that it's very existence aids and abets the gothic horror terrorizing the world.  Think something like Goethe or Marlowe's Faust*, maybe even use Devils (Mephistopheles in Minsc' and Boo's Journal of Villainy is a great BBEG, and even a little inline with his literary precedents); but replace scientific understanding with magic.  Maybe in your world, the Weave usually invoked in D&D mainlines magic's from something like The Far Realm or the Shadowfell or the Negative Plane ... this makes a certain amount of sense in that magic does impact or transform reality as it should work.  That magic usage, the more its used, the more monstrous incursions the world suffers.  This is actually in line with a trope used in a lot of horror TTRPG (I'm thinking of Chill and some of the Cthulhu mythos games) where the more you learn about the unnatural the more you aid the unnatural's grip on your world.  In other words, magic isn't an enlightening force in the world, which is as it is in a lot of traditional "heroic" or "high" fantasy D&D.  Rather it's actually the catalyst of the world being so dark in the first place, whether it's being used with good intention or not.

You can even toy with clerics and druids and Palladins here, if their magics are supposedly divine, but magic is actually innately tied to this more horrific incursion, exactly what are these divinely isnpired classes actually working in service of?  Let's not forget mind bending of the audience, let alone the protagonists, is an important part of good gothic horror.  

I don't know if VGtR even goes into it, but this sort of cosmology I'm laying down may actually be how magic works in the Domains of Dread bound by the mists, or at least it's plausible.

*Faust is an excellent Wizard or Warlock template, arguably an Artificer as well, and the Artificer class also gets all caught up in the web of this "dark weave" I'm laying out.

I'm not sure if this will help but what if low level (say level 1-3) magic users are fairly common place, these would be your traditional wise women, wild man in the woods, holy preacher types, but the only way to advance to the higher levels is by "besting" someone of a higher level. So magic users of 3rd level need to seek out and "best" a 4th level or higher magic user. This then breeds a certain amount of paranoia in the spell casting communities, some may try to hide their power level, other may revel in in and stage magical tournements etc.

EDIT: this would mean that higher level casters also get fewer in number the higher the level you go, so you might only have one level  17-120 spell caster of any given type in a country, with three or four at level 14-17 etc so the act of finding one of these powerful casters is a quest in itself.

"Besting" a superior practioner does not need to mean you kill them but might mean you solve a riddle or maze of their devising to earn their respect, go on a errand for them or could be as simple as crafting a spell scroll of a certain spell.

Due to the somewhat secretive nature of more powerful spell casters the non-spellcasting folk then become a little distrustful and possibly resentful of them. This would allow you to bring in some of the more literary gothic horror, whether that be a spellcaster taking the Dr Frankenstein approach to their work, a Van Helsing type hunting down creatures of the night or rumours of an acient powerful spellcaster wrapped in bandages trying to reclaim their long lost empire.

You can also then bring in more political aspects as clerics try to vie for greater power within their respective churchs, wizards might try subterfuge to win prized positions in court etc.

You don’t just need to use your brains in Gothic horror. You also need to use your heart, your emotions (or at least your emotional IQ) and your morals.

Gothic horror is “suffering in style.” If you do that right, you’ve got gothic horror, regardless of anything else.

Until a caster gets a couple of spell levels up, they’re basically just a dude with some extra tricks, maintaining the feel you’re going for will probably be more about slowing down the leveling both for PCs and allied NPCs. Perhaps, as Rob76 touched on, going above a certain level requires a significant thing to be done beyond just getting enough experience points.