Once you account for popularity, I seriously doubt Batman is the most derided superhero. Hell, Bruce isn’t the most derided version of Batman, that would probably go to Jean Paul Valley. I’d be honestly surprised if Dick or Terry are anywhere near as unpopular as Jean Paul. (And, if you just had to google “Jean Paul Valley,” to figure out who I’m talking about… I’m sorry.)
Batman’s simply one of the most prominent comic book superheros out there. Which means, if someone’s looking to slap comics around, he’s right up there with characters like Superman and Spider-Man.
If you account for that, there are a host of characters that (deservedly) take far more flak than Bats. For someone unfamiliar with comics, characters like Grifter, Spawn, or Barb Wire are far less likely to be singled out. Even then, in part thanks to South Park, I’d kind of expect Aquaman to be the favorite superhero punching bag, because, “he’s Aquaman; he can talk to fish.” At the same time you won’t see anyone going after Fathom because, “who the snot is Fathom?”
But, so long as we live in a world where Grifter doesn’t have a live action film, the odds that someone unfamiliar with comics will mock him are pretty low.
As for the underlying question, why do superhero comics get mocked? Because the setup is actually kinda goofy. The have a place, and value as entertainment. Some examples are right up there among the best lit of the 20th century.
(If you’re wondering, I’m thinking specifically of Watchmen.)
There is a degree of absurdity baked into the core of the genre. It also predates comic book superheroes, and is there with the pulp characters like Doc Savage or The Shadow. Some of the best superhero stories address that head on. Either as deliberate commentary, or as parody. But, we are talking about a guy that dresses up as a bat, and terrorizes people because: “MY PARENTS ARE DEAD!!!!” It’s not hard to see why someone might find the premise a bit amusing.