The atmosphere of the game was great, sidescrolling platformers aren't much my thing but this has some great potential.
It does seem a little resource intensive, but the main issue seems to be that the platforms and mushrooms are a bit clunky, if you don't stick the landing in a lot of places you just get stuck and have no choice but to fall down.
One suggestion, the thorns should probably one-shot you, and you may want to clear out some of the spaces in the thorn pits where you can just walk/stand and not take damage.
Also, just as a detail, maybe make the character's face animate to show pain when getting hit too, but since it's a SIDE-scroller, it's not that big a deal, just would be a neat detail.
Otherwise, a pretty good entry, and hope you finish it up in the future!