You just turned the cutest pack of villains ever seen by the gaming industry into wretchedly disgusting mosnters. That's really applaudable.
The color pallet here is just terrific. You're using these nasty caustic looking yellow-tinted colors that literally sicken you if you stare at it long enough. The application of color is also very well done, including the shading. I also liked how you put all the characters on the yellow-smoke side of the backdrop, leaving this empty dark area in front. This gives you the sense of an invasion, without using any bright colors that would ruin your pallet.
All of the characters are just terrificly disfigured yet recognizable. My only real complaints are regarding Bowser. The exposed side of his shell seemed concave, essentially jutting inside his back and giving him an anorexic look. Something should be done to give his body girth and make him the menacing, towering figure you intended him to be. The tail is also problematic, as it looks to me to be protruding from his back instead of rising over his shoulder. Both are complex depth issues and need work.
I gave you really high marks because it physically forced the expression on my face to change. It was a unique idea that was very well pulled off and leaves the spectator with a stunned "wow" feeling the first time you see it.