When shading an object, there's something called a terminus, which is typically about at the "equator" of the object relative to the lightsource, after which the shadow ceases to get darker. It may even get lighter if there's some reflected light from the surface behind it.

Good start! I think moving your light source a bit, even if you still want to have it mostly a top left, can help you get rid of the pillow shading feeling a bit. But it's nice that you can tell you already want to have some reflected light instead of a straight gradient, for instance, so keep it up!

Not bad but it can be improved!

I suggest you to use less colors (43 are a lot of colors for this simple slime, maybe try to do it with 8)

If you want, you can look at some tutorials