My DD never would have wanted to do that, but every girl is different (I would say it's a minority that do, but those that do obviously LOVE it!). She also didn't like having her hair done, so it would have been torture to her!
I met one really sweet mom at Disney once, whose little girls had their hair in little buns and tiaras with glitter even I think. I asked her if they had it done at BBB, and she replied, "No, it's the MOM boutique! I did it. Have you seen how much that cost?" We laughed - she was funny! Her girls looked cute, and were happy with their princess stuff too.
Does she like going to a beauty shop? If not, I'd skip it.
I've also seen girls with beautiful BBB do's and gowns at MK, when it was super hot, and some looked like they'd rather be in a tee shirt and skirt or shorts. It's good to pay attention to the weather with those gowns, since some may get hot in the summer.