Goomba's Sprites

[Gors]

Your main issue right now, as you already know, is the shading. The way you're distributing the shades and highlights are making the shapes look like stickers. Notice the constant band of color you did in the shading of some things, like the bush. That is called banding, and it is not a good method of shading. That is not how shading objects work, as the light/shadows should wrap it as a 3D object, not 2D. Using textures can make it look better, as it'll break the banding.

Also the bigger hill is not round enough, like the small hill. Work on it to be rounder.