I don't think it's a throwaway joke. It is actually a pretty clever rhyme. The ladies are voicing a genuine concern. They've always seen entertainments where the women were traditionally beautiful, shapely (in the early part of the 20th century VERY round and voluptuous). Fanny doesn't pay them any mind. She has heard this kind of thing all of her life-her nose is too big, her chest is too flat-it doesn't shake her confidence in the least. She tells us in about 10 more minutes that she is the GREATEST star, she is by far. She has the last laugh.