I think is it part a stage and partly their personality.
Not all kids are that way, nor at that age.
It also has to do with their self-identity (which ebbs and flows throughout childhood), and it also has to do with their own sense of self-confidence and knowing WHO they are.
All I know is, I have always taught my kids to be themselves. Not being a "copy cat." My kids are 4 and 8, and they really just know who they are and don't just do what other kids are doing. They will even say "I am myself." That is what we taught them, since they were Toddlers.
But, yes, it is a personality thing too, and how a child either needs/wants 'approval' of external influences or not. Or tries to be like other people, or not.
Even some adults are that way.
Just guide her. Teach her to be herself. Not just a 'follower.'
Sure, at the Pre-Teen or Teen ages and beyond, 'fashion' is important to kids. But make sure she KNOWS the difference between what is appropriate or not, and what it means to be herself. Not just going along because other girls are doing things or looking a certain way.
No matter what, no matter what 'phase' a girl is going through, you GOTTA guide the child... .and teach them to "DISCERN" others, and to know, themselves. To be who they are, and to be self-assured, and not just a follower.
Also teach her, that the way she presents herself.... can be appropriate or not. Even for adults.
When I was a Teen, sure, fashion/dressing/looking nice or hip, is a phase you go through. I did that too. But there is a difference between it being a 'phase' or if it is a preoccupation... that makes the child insecure or more in line with their own values and identity, and which is healthy for their well-being.
My own daughter is 8 and will be 9 in a few months.
She is girly but not always. She likes to fuss around with her hair and nails too. But, even she will tell me "I dress for who I am... I don't have to dress like my friends or those sassy girls...." She knows the difference. But we let her play around with her clothes and nails and hair. Its part of growing up. But, of course we teach her, that that is not all there is, that makes a person who they are.
Girls, just like those things. Hair, nails, clothing and trying to see what looks nice etc.