and, maybe, I think, tied into the fact that chances are the characters we are writing about are depicted in canon as more than commonly attractive, so even if we only think our own bodies are 'okay' or 'average' the characters are probably hotter than we are -- so the only way to give them body image issues is to focus on wounds-as-imperfections.

Yes! I think that's definitely part of it. I mean, it takes a lot of doing to make John Sheppard hideous. And so we have to resort as writers to a pretty extreme injury or disfiguration in order to convince the reader that this body could be viewed as ugly.