There are plenty a mini skirt that don't show ass. Her ass was hanging out, its borderline indecent exposure so I think the guy wasn't necessarily wrong to say something, he could've been more eloquent I guess. To say don't take pub trans or jump in a cab if you're scared of ass is pretty idiotic and ignorant dude.
I have kids and if we take public transportation to a ball game or the museum and saw that, I might say something too (after I got my peek in, naturally). Same way I currently tell young punks to watch their fucking mouths around my children. If you don't care what your kids are exposed to, I feel sorry for them.