I do think girls of about 6/7 can go through a very self-critical phase. The Toddler Taming guy (Christopher Green?) says that up til 5/6, children don't really make social comparisons, they then go through a massive realisation they are not 'the best' at everything. I found around this age (6/7) my daughter started saying 'I'm stupid' and 'I'm rubbish at everything' or even 'I hate my life'. She is not so focused on looks, but again, the odd thing which others have said bothers her.
BUT, the key advice was not to make too much of this. I've tried to help her see that just because you do one wrong thing in a day, doesn't make you rubbish at everything. It's working great, and she's now better able to evaluate her life in a much more realistic way.
But little girls can be over-dramatic and the best thing is to play it down and deal with it tactfully, not attract as much attention to it and publish it in a newspaper!