You really need to work on shading actual forms inside the outline rather than just adding shadows and highlights around the edges. This is probably one of those times where a reference would be a good idea-
(I've never been terribly good with organic shapes and I'm not happy with how that turned out but it gets my point across)
This came up in one of your other threads, too. Instead of starting with a line art, filling it with a midtone and then adding bits of shadow and highlight, start with the darkest color as a foundation and then add forms on top of it.
Now, I know what I made up there is realistic, and it doesn't have to be, but you should still pay attention to the structure of what you're drawing whether it's stylized or not.
Also, regarding the palette, try not to use straight ramps of 100% saturated dark ____ to 100% saturated light ____. Shift the colors just a little bit and have your darker shades be a less saturated and the colors will be a lot more interesting. Also, I don't think I've ever seen a straight grey skull.
