Very common problem with the girls of this age. Starting in the pre-teen years, for the girls, the body becomes an all consuming passion and barometer of worth. Self-esteem becomes too closely tied to physical attributes.

Here is a very good resource for how you can help your daughter develop healthy self-esteem:

Sounds like outlash from being bullied herself. Age 12 is kind of difficult but one thing I would do is maybe take her shopping and give her a certain amount of control over what she chooses. This will help her in making choices and then feeling good about herself. Plus she has control and is not having to be controlled by bullies. Next, I would have a meeting with the school and then send them a letter requesting help as well. A letter is usually not ignored. Finally, I would talk to your daughter about self image. Sit down and ask her to give you 5 good things about herself to you. Then after this, have her meditate on her favorite. All human beings are made of both weaknesses and strengths. How can she strengthen her weaknesses? Sometimes a diary will also help.
Good Luck

I had low self esteem when i was her age and i kinda just got over it. I went to this teen group in 6th grade called Teens Climb High. It's mainly for teen pregnancy but it helped me find out who i was. then just last week i was just roaming the internet and found this website. It's called Girls Rule Forever. It's an empowerment for girls just like her. You should have her check it out. Tell her this to. There's always something your good at. She just hasn't found it yet. True beauty comes nowhere but the heart. Whenever she does something in school or out of school try your very best to be there. I barely had friends when i was 12. It was only because i was scared to reach out and try to make friends. If she wants to be like all those celeberties show her this youtube video called Dove. It show's what it really takes to be them. I hope this helps. If it doesn't then show here that you and your family loves her the way she is. no matter how she looks.