Fun idea with a good execution. Platforming could have used a more dynamic jump where it's higher the longer you hold down space. Mushrooms could blend in to the background because of the similar color pallets. And all the enemies seemed to act the same, if they behaved a bit differently it could have made the game a lot more interesting.
I'm not sure if this is a bug but some levels ended as soon as I got all the collectables, and others required me to go all the way back and talk to the dude at the start. Other than that it was a solid game that made good use of the theme, I liked the art and music in particular. Nice job!