I guess it depends on your approach to being "that girl" what is the motivation for it? Is it to show the world who you are and be idolized or is it because you know that you are a representative of God and that you just want to do him justice. If one does it for the first reason, then that is empty in my opinion because you are apeasing the World. If one does it for the second reason, then it would be on the right track until you consider what God sees in us. God doesn't see our hair/outfit/material possessions as being anything all that. he looks and speaks to our spirit and if we don't first get our spirit right with God, it wont matter what our outside looks like because even if we fool the whole world with our 'that girl' projections, God knows what our motives and intentions are on a spiritual and heart to heart level.
I personally struggled with this for a long time. I wanted to dress a certain way, wear my hair a certain way, lose weight and all sorts of other things, but when I talked with God about why I wasn't succeeding at these things, it was because my motivation was all about having other people see me, rather than to see the light of God in me. It is still a struggle because that mentality gets within the fibers of who you are and you have to rely on God to get it out. I have no doubt that God is working on this in me and that as my motivation for wanting to better myself changes to please God rather than the world, so too with my own perception 'that girl'
As a Christian, what I see as 'that girl' is the women in the Body of Christ both married and not married, who spend their lives supporting causes, spouses, families, children, still find time to be active in ministry, carry themselves with the spirit of Christ-likeness that is easy to spot, have a prayer life that is a testimony to the glory of God.. . .and they still look fierce and energetic while doing it all. The ones who can give you a word of encouragement from God himself in your time of need and recommend a fly outfit for the church picnic too. lol That is who I strive to be as 'that girl'