Yeah, it looks like basically all you did was color in some line art, with a few details of course. Like you filled everything with a midtone, and then put simple regions of light and dark. That's not how shading works. It conforms to shapes on a surface, just like the hat and the fish tail in Seiseki's edit. When you hold your hand up in front of your face, there's a lot more going on than |light|-|mid|-|dark|. And once you start working with shadows over details, you can give the illusion that details are there without really drawing them. In a small piece, you have such a limited area that you need those illusions, because you physically can't draw everything.
But yeah, it does just look like a doodle. Most of the line art is unrefined, especially in the helmet area, and too much of it is basically cartoon shading. Which is to say not much at all, and what is there is very basic.