It's no secret that girls are very critical, sensitive and concious about their own appearance.
Even if they have the "perfect" body, they will pick something that they dont like. Being critical of another girl in my opinion is a way of making them feel better about themselves. Pointing out that someone is "ugly" will help them think within themselves that there is someone worse off out there than them. Notice how they say it to their friends? They are looking for reassurance.
