This is specifically about the area Thundertree from the Lost Mine of Phandelvar.

My players had a novel idea to try and gather up the zombies in the town and lure them to the dragon.  This was at the end of last session and now that I have had some time to look into it, I almost feel like this was an intentional way to 'solve'  this area.  The zombies and twig blights are slow at 20 ft, the zombies say they will follow them as long as they are still in town, I assume that means if you run far enough away you can ditch the twig blights without fighting them much.  Hastily search buildings that don't have obvious enemies, The spiders in ambush slow the party down enough with their webs for the horde to catch up.  Get to the dragons tower and you could have a large number of zombies in tow.  Dive in, cause chaos, get the dragon having to deal with zombies munching on him.  Then hopefully not getting murdered by zombies when the dragon leaves.

I guess it would make for a very different scenario, more of an outrun the zombie horde, the larger the horde the more damage dealt to the dragon.  Curious to see if they end up following through.

From a narrative perspective, I think that's a feasible way to solve the dragon in Thundertree. In fact, I love the image of a horde of zombies dog piling onto a dragon trying to tear it apart. 

That being said, I don't think it was the intention of the module. By the numbers, we'd have to increase the number of zombies in Thundertree tenfold to do more than a scratch to the dragon. By my count there are less than a dozen zombies in Thundertree and around twenty twig blights.  A horde of 30 monsters is intimidating to a tier 1 party, but a young green dragon wouldn't have a problem taking this entire mob out with a couple breath attacks and its three melee attacks per turn.  That's not even including the dragon's cultist allies that live nearby. 

It's a fun idea though. Really curious what will happen if they commit to it!

The breath attack of a green dragon is poison... The zombies are immune to poison.  So the dragon has to kill them one at a time with melee attacks.  Whether the zombies deal all the much damage on their own they still provide a substantial distraction.

I'm aware of the poison immunity. The poison breath would be to disperse the twig blight mob and potentially try to catch the PCs in it as well. I agree it'd be a distraction, maybe I'm misunderstanding the intentions of the PCs--they want the dragon to run away right? I don't think the zombies are a big enough threat for a dragon feel it must flee its lair. Or is their intention to slay the dragon after it has been weakened by the zombies? Because again, I don't think the horde will actually inflict much damage to it. 

Twig blights likely will not factor into it, there is nothing compelling them to continue chasing, but there are 11 zombies in the town, about a third of them will hit the dragon in a given turn.  Not to mention everytime a zombie is hit for the first time it does the ash puff with a chance to give the dragon disadvantage on most things (it is not a poison effect so it is not immune).  The Dragon does have a high Constitution saving throw but it only takes one fail to put him in a bad spot for a round (will likely succeed on the save the following round).

The dragon runs away at half health if able to, so this is more of a good start to combat giving a couple rounds of extra attacks from the adventurers.  Which is fairly significant, in what players can do with a few semi free rounds.  Even if one or two goes down to a breath attack they can recover.