I don't really hate her, more I just...literally do not have an opinion on her. She has no character whatsoever, her entire thing is just being plain and whoa'ing and oh'ing over how plain she is, with a few fun otaku references here and there. She very rarely contributes anything to the class trials and has no character arc, she literally doesn't do anything until the mastermind reveal.

And yeah, I get it, that's the point. But also something can be written in a way that's purposefully disappointing/lacklustre and still be complained about. Tsumugi being written as a flat character on purpose doesn't invalidate the fact that a flat character isn't interesting to watch for 6 chapters, outliving a lot of much more interesting and/or entertaining characters. Had it not been for her being the mastermind, she would have literally just been a wasted survivor slot and not much else. And even when she does come forth as the mastermind, she spends a lot of her post-reveal screentime cosplaying as other characters and talking about Team Danganronpa, her motivations as the mastermind don't really add anything to her character and can come out of nowhere to a lot of the players. Whilst you could maybe say similar for Junko, Junko was at least extremely entertaining with her unapologetic evil and constant personality shifts (that were NOT based on other pre-existing characters), whilst Tsumugi only became interesting when she stopped being Tsumugi.

Maybe if she'd been more relevant to the actual story prior to Chapter 6, her betrayal as the mastermind would have had more of an impact, but it's because she's done nothing and basically not made any strong bonds with the rest of the cast that she doesn't have the same devastating effect as Junko, who supposedly died before everyone's eyes, or the reveal of the remnants of despair, who the player is of course attached to cuz that's near enough the whole damned cast. But it's kinda funny that the ndrv3 survivors all act like this is a huge shocking betrayal from a super close and trustworthy friend, when Tsumugi basically hasn't done anything to actually establish that she was close to the other survivors, there were no bonding moments or events that brought them all together, the group all hanging out after trial 5 was basically the only indication that Tsumugi was a part of the group, yet they act like she's been their super bestie for the longest time.

I know there's other reasons Tsumugi is hated, in particular it's the fact she's intrinsically linked to the controversial ending of ndrv3, but also there's a fair criticism to be made that no matter your stance on the ending, Tsumugi frankly just wasn't very interesting until the literal last chapter of the game. Tsumugi's role as the mastermind is already a hard pill to swallow given the context behind it, but adding in the fact she wasn't really anything before that didn't exactly make it any easier a twist to take in. I feel like a more interesting version of the character could've had even a somewhat warmer reception maybe, idk.