I think the most important thing you can do is focus on the rest of the world and the value we put on a specific type of "beauty" and focus less on her and how she's perfect. She is likely not perfect in the eyes of the world and when you say that, it's her MOM saying it. Of course you think she's perfect, she will tell herself that even if she doesn't express it to you.
I've had ongoing conversations with my girls about this for most of their lives. Here are a few points we've talked about:
There are so many examples of very successful women who are not conventionally beautiful or super thin. Point them out and talk about them.
Some of the most "beautiful" people can be very ugly inside
Some people assign a lot of importance to looking a certain way. Do you think that's right? What type of person would you prefer to be around, superficial or deep?
Point out the air brushed craziness around her, find the "mistakes" online. It's funny to see what they'll do in the name of beauty that ends up looking totally weird.
Make sure she has lots of ways to show her beauty to the world. Being involved in sports, activities, being a leader, a great student, a kind person. These things will boost her self esteem, keep her busy and keep the focus off looks.
You're in for a ride the next few years. In my experience the focus on appearance intensifies through middle school, especially for girls. It seems for a few years many tweens latch on to society's rigid scale of who is attractive and who isn't and it becomes very important.
Then something funny happens around the middle of HS, they all grow up a bit. Suddenly the awkward, chubby girl becomes the valedictorian or prom queen. The too tall, skinny girl develops into a truly beautiful, confident woman. The wallflower finds her footing and all the boys start to notice her. They start to care a little less about whose hot and whose not and a little more about what's coming up in life. In short they start to see past 12th grade to all the possibilities in the world and realize differences in people are interesting and attractive.
Help her see that all this is just a moment in time. It doesn't define her or who she will become. Help her to stay focused on what is important to her. To stay healthy and choose good people to be around. Most of all remind her she is not alone. The ridiculous race to physical perfection is like a hamster wheel. Focus on the real stuff and true beauty will follow.