Elastigirl worrying about her bum makes no sense
This should be self-explanatory, but.. it's Elastigirl. She can mold her body into a BOAT, for crying out loud!
Why would she EVER worry about any shape or size her body is in, when she can ALTER IT so easily to anything she likes? The mass remains the same, but how that mass is shaped, is completely up to her. Instead of thinking her butt looks too big or whatever, she could just add an inch to her height or do that kind of shape adjustment.
Is there a limitation to her power? The movie doesn't tell us. (This is why movies should tell us things, so we wouldn't have to speculate and wonder!)
If there is, then the kids were in danger in that boat-scene.
If there isn't, then the 'buttocks' can never be a problem to her.
I can understand this type of a joke in the old Disney movie, 'Peter Pan', where Tinkerbell saw her mirror image and started worrying about her 'curves' (as materialistic as it is - and as if she never saw her mirror image before.. she would've seen a thousand mirrors already by that point, but never mind), because it makes sense in many ways.
However, of ALL the characters to repeat this joke, Elastigirl is the WRONG choice, as she is the ONE character that NEVER has to worry about this sort of thing, because she can always 'slim' (or the opposite) by changing her body shape!
The whole thing makes no sense.. it would, if it was someone else, you know.
BUT ELASTIGIRL?! WHY HER?!