Well done Grynz, just a few critiques:
1. The koopa bosses arm seems to be growing out of his chest.
Maybe make the shell in front on it.
2. I agree, a few less colors, and be a bit more consistent on the shading and direction the emoticons are looking toward.
3. This one is merely just an idea, try adding a simple grass/sky (or whatever you want) as a background to the hoping bunny, with some type of flower (or clouds) that steadily move to the right, as if he really was hopping somewhere.
Good job, hope to see other works from you in the future.