I'll try and answer this but it's complicated...

From what I gather, there are a bunch of different groups that dislike her for different reasons. Some people are just misogynists with fragile egos who can't take a joke. Some are prudes who think less of anyone who does sex work. Some dislike her political comics for being "low effort". Some think her comics are unfairly boosted (bots or something I guess??) to be more prominent than they should be. Some are upset over her apparent misandry in one particular comic (and the doubling down defense of it). And lastly some dislike how much "power" she has to quash all criticism/critique/parody of her comics.

It's messy because the ones in the last couple of groups get lumped in with the ones in the first groups and it gets vitriolic/toxic pretty quickly. You know, like the internet tends to do.

Is there like a specific instance of her doing something?

I've read the following arguments:

• her humor is bland, Cathy-level stuff. I like [email protected] so I'm not exactly an expert on humor, but fwiw I like her art style and I find her strips funny more often than not. That sort of thing's subjective anyway.
• she had reddit comics-strip mods delete any critical comments about her. Yeah, reddit sucks, that's why we left, I dunno if it's fair to blame her for that.
• she supported reddit during the API protests. I kinda disliked her for a while because of this, HOWEVER since then she's done a lot of anti-MAGA comics (most recently like this, this, and this) so I think she's cool bc that's a far more important issue, and I'm not gonna be hung up on reddit forever.
• in her strip she sometimes makes fun of people who criticize her. I see this kinda like a stand-up comedienne dealing with hecklers, i.e. it's part of the show, and good on her for not taking shit from people. But some people apparently think this is inappropriate. Others assume that her "hecklers" are sexist, insecure, etc and that creates even more defensiveness. Not sure if there's an easy solution to this.

I've commented on her comics on Reddit years ago. She was a defensive crankpot. I'll call that, bitchy.