My daughter likes to wear jeans that are so tight I really wonder how she doesn't suffocate in them. And because she doesn't have a small frame, the skin tight jeans make her look heavier than she is. I've tried to tell her that her jeans don't flatter her figure but she basically tells me to mind my own business because they're what's in style and she likes them. I'm embarrassed for her to go out in public in them but I really don't know what I can do to help her at this point. She's 13, by the way.
I think it's a teenage girl's rite of passage to wear clothes that look horrible on her simply because they're in style and everyone else is wearing them. The more you nag her about her jeans the more she'll want to wear them. Not to spite you, but because she thinks you're dorky and don't know what you're talking about. I say leave her be and let her wear what she wants. One day she'll look back at pictures and be appalled over how she looked- we've all done that so why not her? However, you can attempt to steer her towards more flattering outfits when you shop with her. Stick with being the expert on skirts and dresses and leave the jean choices to her.
Good luck, and look away!