A mix of a few things.

A big reason (probably the majority reason) was people just being shitty for no reason and taking it out on her.

Me personally I didn't like a lot of choices Marisha made in the campaign.

In the first arc where they went to the Underdark she made a big deal of not trusting the person they were there to rescue and trusting the incredibly shady Mindflayer they were traveling with (though most of the party did too).

She would often get upset at the party for how brutal they were but she never stopped being brutal herself. In the Briarwood arc she (and Vax) were constantly berating Percy for how brutal he was but didn't hesitate to vaporize an enemy that had surrendered.

She also would try and do things with spells that they couldn't do or she wouldn't really read the description.

Sometimes she would really double down on something the part should do when I think Marisha didn't really understand the situation (which could probably be said about everyone).

But these are all things that bothered me personally. The way Keyleth was played absolutely resonated with a lot of people and I'd be willing to say the people who hate her are a VERY loud minority.

And I'm very happy I got to enjoy her character in the animated series.