I think it's probably just that the actor is the face of the character, and these kind of people aren't smart enough to either separate the character and actor or to think about the fact that it's the writers who decide how the character behaves, not the actor.Even if someone doesn't like the show or the shows portrayal of a character (and I personally like the show and Galadriel, even if the show is objectively a bad adaptation), harassing an actor that has literally no power over the scripts is just moronic. No that anyone should be harassed, but wouldn't the showrunners/producers be the ones to complain about, not the actors who just work with what they're given?
Then again, if people who harassed other people over stupid things online thought rationally they wouldn't be doing it in the first place.