These are great! I do have some minor nitpicks, though.

-Frisk's hair seems a bit too round on the front view.
-Flowey's petals are a bit lumpy looking. The anti-aliasing between the petals and his face doesn't work either.
-Overall, your colors are a bit too muted and close together.

I did an edit to demonstrate the last point with Sans.

Yours -> Mine


Most of your palette seemed to be from the same hue, which killed the contrast imo. If the washed out look is what you're going for, then disregard this.  (though in that case, you may want to lighten the outlines to match)

Oh, and a super minor nitpick- Sans only does the one-glowing-eye thing during his boss fight, so if you want him to look friendly I'd add the second eye.

Made this zombie guy for an art test. Didn't end up getting the position, but I wouldn't have made this otherwise, so it's still a win for me. 

Anything glaringly wrong here?

Thanks a bunch kullenberg, I had some issues with it but you articulated them exactly. 



Tried to bring the blue/tan into the other reflections, but it definitely doesn't look right. It's probably obvious that I'm just making it up at that point.




Here I just removed the reflected colors altogether, and I think it looks the best so far, or at least less messy than the rest. Which is a bummer, I really wanted to get those reflections in, but I think that sort of complexity might be beyond a small piece like this.


Oh, and I changed the shading in both of them at the bottom to address the "hat" problem. It might still need some work though.

You're actually pretty close! try removing the interior shading on that.

Which one are you referring to?

In the meantime I tried these two versions. Trying to go for a slight bevel. I feel like I may be progressing backwards haha




Here's a version without the shading on the inside, though to me it looks more flat.