The problem with that is they can't take pauses to recurl Anne Hathaway's STRAIGHT hair. They were going for DRAMATIC transformation. Her mother had wonderfully curly hair in that movie if I remember right.
It is based on a book. While I have not read it, the straightening could be what the author wrote.
As for the animated ones while they may not have curly hair (which is hard to continue for the animators, esp. when it was still all done by hand) they have lush, full of body hair. Aurora is arguably a wavy, and Wendy has lovely curls, that stay curly even after flying through clouds (damp!) and falling in the mermaid lagoon. Disney has never been one of accuracy. It's more of the broader message. What does happen to Mia is that she learns that it's not what outside that counts but what is inside a person and being happy with who you are.