Star Trek has been hugely popular and none of the seasons were ever bad. But there are ones that were better than others and some that weren't all that the show stands for. They would have been great for another show but Star Trek has a repuation to live up to and one season in particular did not live up to it. It happened to be the very last season. This opinion isn't shared by all. Some thought it was fabulous. But more were let down by it than were pleased by it.
Why was it so bad? Many viewers felt like it was all too sloppy and that the writers, producers, etc. took the easy way out. Fans felt cheated for having put in so many hours and all they got was a lazy ending. Some, however, did not feel that way. They felt that it ended as good as it could have. Like everything, opinions are always plentiful and rarely ever are the same. Some felt that there were way worse seasons but the majority agree that the last one just seemed a bit lame.