You're allowed to bash a band if you qualify your fucking statement. I didn't think this would be an overly difficult concept to wrap your head around.
If you're going to go out and say 'Mastodon sucks', you might want to tell us why you think they 'suck' considering the amount of critical praise they get from a wide spectrum of well-regarded music critics, both traditional metal pundits and non-traditional. I may not like Megadeath, but I will never say they 'suck' simply because I don't like their sound since I recognize how recognized the band is for the quality of most of their albums within the thrash genre.
So, this has about fucking nil to do with having it both ways.