I think that background characters are bland. The characters who either don’t speak (and are not mute) or say generic thinks and are only there to make the scene look populated or act as shop keeper #1.
Bland characters perfectly fit a troupe. The bad guy is a bad guy in order to fill the bad guy role or someone’s just there to play the rival or lover role and there is nothing unique or special about them.
Boring characters say boring things that the reader doesn’t care about. There are lots of school-based fictions where the characters are almost never seen going to class, because lectures about trigonometry are boring to most readers.
For general Main characters, the story is centered around them and it is interesting for them to have a personality for Character driven Stories. For plot driven stories, like Mysteries or Horror, the MC is less important and you’re suppose to root for the slasher killing everyone or just care about solving the case.
For interactive works or works where you want the player to imagine themselves in the role of the MC, a defined personality can hurt the work. The have been plenty of otome games where the MC was so awful that people didn’t want to continue the game. The ability of the player to define the MC’s personality is a great addition, but the plot and characters should carry a majority of the story’s strength, although funny defined MC’s are always great.
I like Gary and Mary-Sue characters where everybody loves the MC. It’s like escapism and is fun because it never happens in real life. I also think the whole romantic comedy thing where one lead character thinks the other lead is a PoS who should die is still chased after by the other poor unwanted lead, who should try to find a little self-esteem, is kinda meh especially with the turn a no into a yes thing. The dating sim theme of buying your loved ones gifts, answering trivia about them, and stalking them by joining all the same clubs and classes is also meh. It’s nice when you don’t have to change yourself to get the guy/girl and they love you for you (and your Sue powers).