Inappropriate or tarty is as other posters have said is rarely simply one item of clothing.
It's the whole way you look and the whole way you behave. Sadly some where between 13 and 16 a lot of teens seem to manage it, but not at 12.
If your DD is happy and confident and has lots of interests she won't feel the need to join the thick foundation, high heels, black eyeliner brigade.
She'll wear that dress her way, just as DD1 wore a very tight next dress at that sort of age and DD2 wears shorts and vest tops and doesn't look inappropriate.