It's a teenage girl.... Most don't like their body. If she was curvy she would be saying she wishes she wasn't so curvy because then the dress would look good on her. Just affirm her that she is beautiful and if the dress looked nice tell her.
What does it mean if someone is trying on a dress and they say it would look better on them if they had a figure? (This person is a teenage girl who is proportional and of average weight. I'm guessing she meant if she was more curvy...?)