If someone hates the the Mona Lisa and thinks its a pile of crap, even though everyone else seems to think it's the best painting ever made, that person shouldn't have to agree with everyone else and call it "good". To them, it's "bad".

People tend to say, "it's good, I just don't like it" and it's almost always to keep other people from getting pissed off at them or offended. If that person wasn't worried about everyone else, they'd prolly say "I hate that shit, it sucks." (It's bad)