I just don't understand the need for excessive physical description in fic period, be it flattering or unflattering. If you're talking about canon characters, we all already know what they look like. It's not like a character from original fiction where the author's description is all we have to go on. Anyone who is reading, say, Thor fanfic already has mental images of what Thor and Loki and Jane look like; they don't need the fic author to tell them.

I find an excess of description to be jarring and unnatural in most cases. We have imaginations, let us use them!