They aren't nearly as bad as people make them out to be. Most people just know the *** Nickelback songs that always play on the radio. I have several Nickelback songs on my playlist because they're actually pretty good. And when I play them in front of friends, they'll ask who the song it's by because they think it's good. I tell them, and without surprise, I hear "Wtf? You listen to Nickelback?" While completely ignoring the fact that they just said they liked the song.
I think it was a bandwagon effect. Some people started making jokes about how Nickelback is ***, it caught on, and everybody just makes fun of the band and anyone who listens to it. People who've never heard a song by them even say they suck.
I remember when 40 Year Old Virgin came out, people were making fun of everyone that listened to Coldplay. "You wanna know how I know you're ***? You listen to Coldplay." Then out of fuckin nowhere, everyone loved Coldplay and they were considered one the best modern rock bands. It's all bandwagon effect.
I think being from Canada seems to have a negative effect on the band as well. For some reason, people in America seem to like giving shit to Canadians.