So me and my friends have just started to play DnD for the first time, with me has the DM. And I really wanted to do a homebrew campaign with them, but because were all new to this we started with Mines of Phandelver first. I have already made my homebrew campaign that ties in with every character. When they created the characters, all of their backstories involved a king, so I made a story that they fled their homeland and by chance they all came from the same continent, and a already told them and everything's great. What I wanted to ask was, is it bad if I put homebrew NPC's and items tied to my homebrew campaign in the Mines of Phandelver campaign?

Not at all. No one runs the adventures exactly as written in the books. Adding your own flair to the adventure is part of what will make it memorable for your players.

The beauty of D&D is that everything, everything, is adaptable to the DM and their players. 

There's an NPC from LMoP that my wife fell in love with, she decided to have the NPC appear in Curse of Strahd, and now the NPC has become her own PC in a homebrew game at our local gaming shop.

There's also something about adding in pieces like your NPCs into a game that really resonates with players. The more they see that the world includes them, that they're not simply pieces of a board game, that their actions and backgrounds truly impact the world in minor ways, the more they will become invested in your games. Go wild, but make sure it is consistent, and take copious notes so you can keep everything in order.

Thank you very much!

Thanks, good to know it's alright.

Definitely, and as new players, don't worry about getting all of the rules right the first time. And don't worry about sticking to rules that you don't like or don't feel make sense either. As the DM, you are the final authority, regardless of what the rulebook says.

A couple examples from the game I am running (Waterdeep: Dragon Heist)

• I plan on running Out of the Abyss afterwards, and so for one of the faction quests, rather than fight a gazer (which would not have been a challenge for my party), I had them fight an adult oblex (sentient mind control ooze) with some foreshadowing that a demon lord was behind it.
• There is a 1st level spell called that makes an opponent move using their reaction, which technically triggers an attack of opportunity. I think this is cheesy and so I changed it to explicitly not trigger an AoO but instead they get the frightened condition for a round.
• The way dragon heist in written, the PCs have to go to a location to get information, but have no clue that it even exists until the point they are supposed to go to it. One of my players needed help with a backstory, and I so gave her ties to this location. That way at least one person could go "Hey, I saw things like that at this place I work for sometimes"

One of my favorite D&D podcasts actually started out playing the Starter-Set Adventure and it turned into a huge 100+ hour homebrew campaign. It's absolutely okay to use a module as a springboard. In fact, I'd highly recommend it if you're just starting out.  

I 100% think that you should add seeds for the homebrew campaign to come up later. I've done it with Dragon Heist and when I ran Hoard of the Dragon Queen. The players are none the wiser, unless they're the cheating kinds of players that read the module they're playing!!! But when it all comes together they will be impressed with your DM skills and sense of foreshadowing.

That's really what LMoP is for is to be used as a springboard for any adventure just tweak a few things here or there for a homebrew setting.