Mia's was probably thermally straightened, as they would not straighten her hair everyday with a flat iron.

And, in my opinion, all of the princesses have a least a little bit of wave in their hair:
curl blog
curl album

"Your head is like a crown on you.
It is as beautiful as Mount Carmel.
Your hair is as smooth as purple silk.
I am captured by your flowing curls."
Song of Solomon 7:5 (NIRV)