Quertalthough it generally works that way, mainstream doesn't necessarily mean "bad quality". Shingeki no Kyojin's excellence is overshadowed by its popularity which obviously generates controversy and unnecessary drama/equivocated criticism.
trust me, if nobody knew about attack on titan, then no one would have anything bad to say about it.
calling bs. My old housemates made me watch that with them and I didn't enjoy it at all. Perfectly reasonable that someone might criticise it.