It's an incel term.

Mogging is basically when you have some gendered trait which is in some way "better" than that of another person, usually in a context where the person is showing off that trait. So for example, a tall man would be "height mogging" a man who is shorter than he is.

In this case, the trait is her butt, so she's saying "I'm showing off that my ass is better than theirs."