Term can be applied to swim suits.
Excess fabric in the butt area, that is not filled out by the butt of the wearer.
This problem frequently occurs on girls who have no butt and are wearing pants that leave room for girls who do. The pants are usual a cotton material, and are not form fitting. Diaper butt usually causes feelings of self-consciousness and embarrassment. Diaper butt is not desirable and is extremely unattractive.
Guys would say, "Oh man, did you see that girl's diaper butt?