I agree that it was underrated but the movie was still horrible.
There are many reasons for it, especially
the bad writing and bad characters. The primary example are is the Klingon bird of prey and her crew - the two new "Klingon adversaries" weren't even really adversaries, and they both were horrible actors and their parts were completely pointless except for the fact that someone decided that they HAD to be at Sha'Ka'Ree to shoot the God-like being out of the sky.
Another huge offender was the Uhura dance number. What? They a) couldn't think of ANYTHING else? This is Captain Kirk for Christ's sake! and b) 10 grown men actually fell for it.
A third offender WAS the humor. It was good but at too many places it felt pushed too hard, almost like they were trying to make a comedy as opposed to a Star Trek movie.