1. She was a symbol of WMMA and by proxy she absorbed a lot of the hatred leveled upon that institution.
2. She became the UFC's biggest star for a time and brought mainstream interest in the sport from people who normally wouldn't care.
3. No matter how good of a grappler Ronda was, her striking limitations were pretty apparent and her sub-par competition was unable to capitalize on said deficiencies until Holy Holm KOed Ronda.
4. By the time of that KO Ronda had become a full fledged heel figure whose downfall Sherdawgz could root for. Her skills were grossly exaggerated by Dana and Co. Her dominance was built on a house of cards made up of soccer moms and accountants, and her attitude definitely made it appear like she bought into her own hype.
A lot of the hatred she got would have been dumped on whichever woman became the first dominant champion in the inaugural woman's division. There's no denying the rampant sexism of Sherdawgz.
However, she did have a heel personality where she made everything into "You're either with me 100% or you're against me 100% and you're shit." She also viewed all criticism as an attack on her, even constructive criticism, and that included the blindsides in her game which her yes-man coach didn't help with, and which ultimately led to her downfall.
Then there's Travis Browne a dead-eyed prick who crushed old legends using brute force and physicality rather than skill, and was generally disrespectful afterwards- "Bigfoot got lucky and made a name off me" regarding a fighter famous for beating Fedor.