I liked her just fine myself, so I can't say for sure, but I imagine some people probably find her boring. She's really sweet and everything, but she doesn't have a whole lot of personality compared to the other characters on the show. Maybe they feel she wasn't important to the 全体, 全体的です story and was just a distraction from the good stuff?
It's also very possible that people dislike her simply because they shipped Willow with someone else. Shippers can be passionate like that :D
I didn't like her as a character because she was uninteresting. Had no charm like any other character, and the relationship didn't feel like it had as much spark as the others. They way they act around each other just isn't interesting. It's like they just decided "Okay, let's put some レズビアン together and let the les-factor drive the chemistry". Pretty much just a couple whose defining factor is "Look, they're together and they're witches". They wouldn't be considered a decent couple if it were Willow and a ランダム dude, because there needs to be an interesting character on both sides. Tara just isn't that.