At gyms it is considered rude to interrupt other people's workout. Some people do it, but they are considered rude.

If a man interrupts another to critizice their appearance, it is most likely to bully the other men for being skinny. So not good either.

I've never seen a woman interrupt a man to tell him that his clothes don't look good or talk about his body. I do have seen women waiting "casually" at places where they could be seen by the men they like so when he stops, he can chat. Or ask for advice on workouts that they already know how to do... Let's just say that men or women who just want to flirt, can do it without interrupting other people's training.