You're too hard on yourself, I think. The body structure of Isaac isn't that bad, I think the arms might need a little work, but otherwise the colors are quite striking against the black background.

Really, stuff like this is best viewed with some distance, as unintentionally insulting that may sound, it's a bit hard to describe but perhaps it's just better to take the picture in as a whole instead of move your face up close to it and mull through the very fine details.

The monster to the left feels a little unfinished, I assume you dropped the project mid way through its production.

All in all, though, I like it.