Yeah there are good questions that could be asked about why so many girls think they have to resort to making out with other girls or continually be half naked in order to be seen as grown up or "sexy," but I hate when we focus too specifically on any one girl. The criticism should be towards pop culture as a whole and how we value female sexuality rather then Taylor Momsen's decision to wear underwear as clothes.
You get what I mean?
Even if Momsen likes dressing like a Victoria Secret model that's been attacked by a Hot Topic brand of "edginess," that's her choice. If Cyrus wants to experiment with what it feels like to take control of her own sexuality by donning hot pants, then who are we to say she shouldn't? How many of us have gone through these very phases ourselves? I know I dressed like a streetwalker for at least a year when I was a teenager (not a classy one either, but more of the ridiculous variety you see on television). And if Taylor Swift prefers instead to resort to a slightly creepy child like version or herself then whatever.
Bitching about Swift's "good girl" image is just as harmful as bitching about Cyrus' "bad girl" one. Both imply that there is one correct way for girls to express their sexuality. If a woman chooses to reinforce every two dimensional stereotypical male idea of what "sexy" is, or embraces the chaste virginal role with a fiery passion, then she's free to do so. All expressions of sexuality, even the ones we don't agree with, are equally valid.
So calm down outraged internet users. Not everyone will follow in Brittany Spears' footsteps (what does that even mean?) and teenage girls have been pissing adults off for as long back as anyone can remember. The kids will turn out just fine.