Looks like a FUN project
The backflip is a nice feature thus a plus point for the gameplay aspect

But there is also an aspect that should get improved asap which would probably be the terrain. Higher resolution via detail map (grayscale image) in combo with a multiterrain texture shader (like cartoon_baboon mentioned already) can make the whole thing look way more plausible.

But IDK if that is your initial intention since shell shading is actually basic coloring with no blending of colors at all AFAIK.
But I guess that's up for you to decide ^^

Cheers

Frazzle