I can't shake the feeling that Jimin isn't appreciated by this fandom and some of his bandmates precisely because he is so mature and unproblematic. Maybe that's a weird parallel but he is like the eldest daughter in the family who is smart, talented and responsible but everyone around her prefers this empty-headed younger brother who causes nothing but troubles and expects everything to be handed to him on a silver platter
For bts I think it's just because of how they've always been. Them and bighit assigned their places in the group which affects how they perceive each other and it just stuck.
With armys the reasons are most of them like tae and jk whether individually or as a ship, and it doesn't matter what anyone says, 7 times out of them if you stan those two you're likely to see jimin as competition. And jimin is one of the members who doesn't do the whole parasocial thirst trapping fuckboy thing, he doesn't give them yn content. He's calm and sensitive which is very far from the bulked up or bad boy who joins tiktok dance challenges image that armys love.
Kpop has changed. Which means what the fans want has changed. What armys want have changed
Jimin was the blueprint when kpop idols embraced androgyny and stans loved them because of it. Now with overt masculinity on the rise again, he doesn't fit the mode of what they want. He doesn't give them what they want.
You can tell with the sudden obsession with 2014/2015 jimin over the past couple years, when he was bulked up, and acted like completely different from the way he acts now.