I guess the thought is if you're going to wear that dress to try and smooth the contours with the undergarment.
Again, the kid is being a kid and I'll add I completely disapprove of the mother posting this even though she claims the 13 year old was good with it. No one, let alone random Netwebs strangers, should be beating up personally on a 13 year old where she's going to be able to read the negative comments. However, I also don't agree with the mindless support of, "You're beautiful" and don't go changing, etc. She's really porky for only 13 and I think the mother should be shot for this whole thing.
The mother's behavior is outrageous, and speaks to the screwedupness of soccer mom / facebook/ life is good / selfie culture.