Oh my, do not push her to "mature" so she can fit in. Do not force her to be someone other than she is already, especially since she is happy where she is. As soon as she starts to feel like you don't think she is the way you want her to be, there will be a lot of inner turmoil.
Why does she need to have a good time at school and have lots of friends to fall back on? My daughter found out really quickly that the "in" girls were not the ones she wanted to hang out with. Better for her to find the girls who like her for who she is.
My daughter was a tomboy and played soccer with the boys at recess. Sure, some of the girls teased her for not wanting to play with them. But over time, she found the ones who appreciated her for who she was. She only kept one as a friend by the time she got to high school and she is still friends now that they are out of college. There just weren't many that were her type.
Just be sure to teach her how to deal with the teasing that is sure to come. I hated those 5th - 8th grade years with the girls in her classes. When someone teases her, tell her to agree with them. For example, if they say, "Your clothes look like they came from Goodwill," have her say, "Yeah, thanks!" or "Yeah, it's so comfortable!" with a smile on her face as if it is a compliment. Or if they say, "Your legs are so skinny," she should say, "I know! Can you believe these are what my parents gave me!" If she learns to confuse them, with her head held high and shoulders back, they will leave her alone or start to like her for her confidence.