It's been a while since I've seen the movie, but didn't he do that earlier in the movie to an actual man in disguise as a woman? I thought the whole point of the joke was to subvert expectations with a failed repeat of a previous gag, not so much to ridicule the target.
Earlier in the film the actor he punched is female but they switch to an obviously male one when the camera looks down to really highlight how absurd it all is.
I just don't really see anywhere in here a joke about transfolk, just about an old woman being ugly. I think the real joke is Austin is a bit insane I think this is soon after he's thawed right?