I have 3 daughters and 5 neices. I remember my beautiful neice (who is now 25) sitting on the stairs at the age of 3 pinching her thigh. When I asked her what she was doing, she pinched her thigh again to demonstrate how much was between her fingers and said, "Look, Aunt Sherrie. Fat." OH, MY GOODNESS!! She was mimicking what she had seen adults do. Society hasn't gotten any better, either.
I remember being absolutely, paralyzingly sure that I was hideously ugly until I was in my 20s. Looking back at photos I think, oh, my, I was pretty, but I never, EVER felt it.
My daughters are 22, 21 and 10, and even though they're all beautiful, they insist that they're ugly. My youngest has been insisting for a year that she's fat. It makes me crazy, but she's picked it up from her friends and, unfortunately, her sisters. I have never been one of those moms who tells their kids to avoid the dessert or "you'll get fat," I don't get on them about their appearance at all except to make sure they're clean, and instead of finding that to be a good thing, they're sure I'm hopelessly out of tough. They're sure that I should be appearance obsessed, and that I'll "never understand" because I'm not. It's very frustrating.
If you're patient, spend more time talking about her talents and abilities than her weight or looks, and encourage others around her to do the same, she'll eventually leave adolescence and her crippling self consciousness behind. Don't expect it to happen until she's far past her teen years, though! Hang in there.