Feminism to me isn't about rejecting femininity. Girls and women are amazing BECAUSE they're girls and women not in spite of it.
In addition this is perfectly normal child development, she's exploring and learning who she is and that includes her sex and gender. To stifle that exploration would be deeply unhealthy.
My dd around the same age insisted on going to nursery in a pink fairy outfit for a week! She was also at this time completely obsessed with motorbikes & helicopters!
She's now almost 18, working full time in a male dominated industry, more than holding her own and hates pink, but still loves getting dressed up for a night out and embracing her womanhood when it suits her.
It's likely a phase, but it's for her to determine how long it lasts. Not you.
Confidence isn't mutually exclusive with femininity either.
You praise every aspect of her to build confidence. Her intellect, kindness, practical abilities, looks... All of it. If you ignore the looks side she could well think you don't think she looks nice which could be just as damaging. You teach her we can be anything - and that this can change on a daily, hell hourly basis!
I have an absolutely stunning friend who people assume is a "princess" type, she is a very successful tech developer and does free climbing as a hobby. She has days when she's working in the day designing some new robot or some such, then climbing after, then later dressed up to nines for a girls night out.
If you want her to believe she really can be ANYTHING she wants you have to back that up in your actions.