I agree with $ZeToxiiK, back seems unfinished and boring aswell as the sides probably.
About the style -
in my opinion it's somewhere between cartoon and realism - the shading is too realistic for a cartoon set and the lack of different textures, details, noises, colour-variations is too cartoonish for a realistic set.
Remember that mirroring a set is not the best thing to do.
The idea is most important - think, search for inspiration, imagine and the drawing will become much more interesting.
The design is very nice but you should try to add some details, characteristic things, maybe some kind of pattern, texture or other colour? I also like the mapping.
But if you wanted this set to be as simple as possible and you don't like mess - keep it clean and simple as it is now
Overall - except the blank spaces and a little bit boring back the set seems decent so far - but you can easily make it much more better.
Btw do you have any head in mind? Or a reference image?