My 11-year-old daughter has just started scondary school and has been coming home with comments like this about herself. She is very pretty and it was catty comments from some of the girls at her new school which caused her to feel this way about herself. I managed to convince her that it is jealousy, and that when they tell her she's too thin, or too tall, her teeth are too big or she is too rosy-cheeked, it's because they wish they were like that. She now finally believes me and her concerns have stopped. She realised that the girls were jealous and she actually feels sorry for them now.
Its very true...girls can be mean and jealous at this age. And usually they all feel the same way inside.
I would keep a distant eye on the situation and not make it your main concern. As dad I would find something more positive as your main concern to divert her attention. I would say this kind of thing could last for the next 3 years. Good luck!