Here's a comment from u/Useless_as_RCTL from two years back in a similar thread, that I believe sums it up quite nicely:
Like others have mentioned, it started with her writing in a TON of Mando influence and some Grand Army of the Republic numbers snafu. She was firmly.on the side that said the whole army was 1.2 million strong, after Taun We's line in AOTC. The math behind that is... Improbable. That means there'd be like 1.something clones per planet in the republic, and the whole army could be transported by like two dozen acclamators or something. Whatever.
The issues grew when the Jedi characters she wrote all seemed to grow to believe Mando morality was far superior to the Jedi code. It wasn't presented as a nuanced, one individual sees a new way and thinks it's good, but a sort of universal "Obviously this morality is superior" kind of way. I think there's room for great storytelling about a Jedi who sees the flaws in the republic using cloned troops with no say in their creation! But that wasn't what she did.
What really caused the fallout was her response to it all. She referred to her critics as the "Talifan" and it went down hill from there. I think some accusations of sexism and homophobia were thrown around, it was pretty toxic. She had a solid contingent of loyal fans (this was back on the days of starwars.com forums, I was like 12 and surfed them every day). She was pretty active on the forums for an active author, which was cool! But online forums can be... Not nice places, and she definitely contributed to that.
I remember in one thread about a new release, I asked a question about two different Mandalorian dances or somesuch, I don't remember the specifics. I very clearly made an error- it was all in the Mando'a language and I mistook two things and so the question I asked made no sense. She proceeded to kind of belittle me, saying questions like mine are what drives authors nuts because people don't actually read their work. I was 12, and pretty upset by it.
Her last few books were... Well she saw the criticism that she was too partial to the mandos and went HARD on the other direction. All of a sudden mandos were a major galactic player on par with the New republic and Empire. Their ships had impenetrable armor. They were all unbeatable warriors and sources of profound wisdom. Jaina solo specifically says at one point that she needs specific tutoring in how to kill a sith. Who does she turn to for that training? Not her uncle, the man who's killed more Sith than anyone alive. But boba Fett, inexplicably. He turns her over to some minor character Mando's who train her in techniques like sucker lunching and "battle rage". It was cartoonishly over the top.
Oh, and the backlash on Mara, from what I recall was more about her not calling/talking to Tim Zahn at all. He was surprised when the character was killed. Is that Traviss' fault or Del Rey or Lucasfilm? Idk. But right around then the Clone Wars show retconned all her Mando world building so she broke ties with Star Wars, never to return. I wasn't sad to see her go.