It's...cute, I guess?
1) Composition: The cropping is very odd, since you're suggesting that the skulls themselves have expression. While the way you have it does suggest a very cramped, trapped feeling, you can also suggest that with a background (by, say, having the background of the picture be a closet or something).
2) Head/torso/limb proportion: Even taking into consideration your style, the proportions do not look "right" in my opinion. (And by "right" I don't necessarily mean "factually correct", but "makes sense".)
3) Face proportions: TF2's men have Manry Manjaws of Manriness. Faces should clearly, well, face forward, with the rest of it tapering off to the sides, like a box.
4) Proper shading techniques: Either keep it flat (since that is also a style) or utilize proper technique to suggest the volume of the object. Even today's super low-budget animated series has at least 3 shades: the base color, the highlight, and the shadow. But for better definition, I suggest a 5+ shade coloring method: dark/black outline, base color, a medium shadow, a deep shadow, and one or two levels of highlighting.