It's more common in literature set in the past: Mary in A Secret Garden is trying to fatten up, and skinny Anne in Anne of Green Gables is jealous of plump and dimpled Diana.

As for women writing women looking in the mirror: Diana Gabaldon has her protagonist look at herself naked in the mirror and evaluate her own attractiveness, ahead of hooking up with a guy, and wish she knew what her mother's backside had looked like when she was alive.

I know I'm not neurotypical, but do women really stare at their naked bodies in the mirror and mentally compare their backsides to their mothers'? That scene always stuck with me because it was such a moment of "Either I'm from Mars or you are, I'm not sure which," for me.