yuck...
pretty much random mirrored lines without any clear purpose, i find this art style v lazy and an easy way to profit but it's your first set i guess
the head lacks even a contour of a face or something, which means you could have pretty much gone in any direction with the shading and you picked the worst one. plain drop shadows and inner highlights with black outlines? i don't think that regular super simple cartoon shading style goes well with something that you can't even tell what it is
if you're gonna go for a minimalistic look with shading, leaving out the black outlines and drop shadows would be a good way to go imo (though that might be preference idk, it looks really ugly to me)
the arms are ok-ish, i was mostly criticizing the head. i still fail to see what they're supposed to represent though (random lines)
you should work on having somewhat of an idea of what you want your set to be, instead of making something completely abstract with the intention of shading it like you would shade a clear figure/thing/whatever, it's bound to be a clusterfuck of styles that don't go together very well. look at the parts of the set you didn't make for instance, they're simple yet they look better than your colored/shaded thingos (imo)
using gradients for shading would be ideal with this kind of style i think, for your level at least. without the black outlines of course
also don't pull the "i prefer cartoon over realism" card especially when it doesn't look like you can even pull off basic shading yet