Jane Norman dresses are really tarty, they don't seem to do anything that isn't skin tight and very short. I wouldn't be happy, but could be alright with leggings and a cardy?
Of course a 12-year-old can look tarty if they are wearing inappropriately adult clothes, what a bizarre thing to say. Doesn't mean they are tarty, of course, but they can certainly look it.
It's your job to disapprove!