While I don't doubt that for some it really does come down that, I would argue that for a lot of people it's actually Marinette's framing that's the bigger issue than Marinette herself.
The show doesn't really treat Marinette as a complex character. Her woman wrongs are treated as little oopsies that shouldn't be held against her no matter how many times she repeated the mistake without learning anything. Her treatment of Sublime, for example, is basically the same treatment she initially gave Kagami but with a nicer coat of paint. Yet Sublime wasn't allowed to dislike Marinette for it or get angry about her prosthetics getting broken by Marinette's carelessness, because only Bad People dislike Marinette. (The episode even said she's her best self as is, which is pretty egregious lol.)
The show doesn't acknowledge her girl failures either. At this point Marinette failed in almost every goal she had, yet she's still called the greatest Ladybug ever. She lost all the miraculous and Paris came out to cheer her as if she just defeated the villain. There was no public doubt about the heroes' capabilities despite that being very justified after such a major loss. She lost to Gabriel because she stupidly detransformed in front of him, yet everyone acts as if she defeated Monarch (even the writers. In some WoG commentary they outright say that they consider that Marinette won because she got through to Gabriel).
I love me some morally grey characters and people who have something deeply wrong with them lashing out and making bad decisions. However, I can't enjoy what they're doing with Marinette because there's none of the fun stuff with those kinds of characters. The show believes that Marinette is a noble hero taking everything on her own shoulders to spare the burden from her loved ones.
Even Sadnansi, which is the only time the show did an actual corruption arc with Marinette and showed her giving in to her character flaws, declared that the flaw Marinette gives into is that she loves Adrien too much. That's like saying "I like to work too much" as a weakness in a job interview lol.
There's also how people are rightfully skeptical that the show will give the entire arc a proper payoff, considering how they fumbled the s4 and s5 finales. This kind of arc is only fun if the bad decisions actually have consequences and come back to bite the protagonist in the ass. If everything is forgiven without at least some effort on the protagonist's side, what was even the point of it all?
It gets worse if you like characters like Adrien or Alya. Thinking about it from their perspective, they should be seriously pissed at Marinette for some of the stuff she pulled. Yet there's a high chance their anger is going to get glossed over or downplayed for the sake of Marinette not experience too serious consequences.
For fans of those characters, it feels like Marinette got off scot free at the expense of their favorite. Both in terms of in universe, but also in terms of the character not allowed to act according to their own agenda and internal world. They're robbed of agency and reduced to a plot device to service Marinette's arc.
A lot of those things makes her unfun or even outright frustrating to watch, all of which generates resentment for the character and makes people hate her.