Batman 89 wears what appear to be form-fitting leggings, i.e. tights. Most people in-universe have not seen the costume up close.Kindly stop assuming I'm a moron. Yes, obviously it's a joke, but a joke only works if the audience understands the reference. That is my point. A joke about superheroes and tights works for us because we live in a universe where comic-book superheroes are known for wearing tights. So it's incongruous for that same joke to appear in movies and cinematic universes where the actual superheroes do not wear tights.
In fact, most people in-universe have never seen the Batman. But the fact that they wouldn't get the joke doesn't undermine the Joker's speech. It's not as if there's a big audience laugh reaction that needs explaining. As with all of the Joker's jokes, it's for his own amusement, not anyone else's.