Hey salesperson, while you personally may prefer the way a dress looks on you with some sort of smoothing contraption under it, you don’t NEED to wear it that way. I may prefer the way I look with out said contraption, or hate the way the contraption feels so much that I’m perfectly happy to go with out it, and therefor I don’t NEED to wear it. And a teenage girl who’s tall and fit CERTAINLY doesn’t NEED to wear any such thing.
Is a nipple hanging out of the dress? Yes? Well then she might need a pasty because there are places where it is illegal for her to run around with her nipple hanging out. Please learn the difference between your preconceived notions about how people ought to look and what people actually need. Because your unsolicited opinion about the way someone looks can in fact be harmful, and even if it isn’t it’s mean and hurtful. So cut it out.