Pixelart does not mean drawing with large pixels (that's an 8-bit fanart theme). You also have mixed resolutions which is quite a no-no no matter what you're aming for. A background is redundant in this case (PJ prefers sprites etc. to use a transparent bkg if it doesn't add anything to the composition). Welcome to PJ!

 Thanks for the critique. This is actually my first pixel drawing, well first decent one, so no those things you pointed on were not conscious. I've been working on my shading, and have had a loot at the noobtutorial thread in the forums. 

An interesting idea and a fresh take on the old Mario mushrooms. However, I do think that some of the half- and quarter circles look like triangles unless you zoom in. This is confusing. Try to flat them out a bit, like the top half circle.

I also don't understand the way that you've shaded the top of the mushroom. It looks pillowshaded. If you're not familiar with that term, do look it up. You've added shadows to the side of the mushroom stalk, but not underneath the top. Conscious choice?