I don't hate her as a character, I hate her as the unnecessary third wheel she was most of the time.
Haruka and Yuu already have romantic feelings for each other (tho they pulled that anime card "I don't understand what love is", which is nonsensical, really hate that trope). Mitsuki's involvement amounted to nothing in the end (like in most love triangles). And the romantic development that has happened happened on it's own between Haruka and Yuu with nobody cockblocking(lol) them, no "picking". Cut off Mitsuki's involvment in the whole thing and they will be standing in the same exact place as they are now. It was also so forced into the picture as others have pointed out, There was a lot of hate towards her due to all this... Also trying to steal Haruka RIGHT INFRONT of Yuu was a fucking DICK move.
Personally hate love triangles because they stop romantic development from happening. Making chapter after chapter of running around in circles feel like a massive mindnumbing drag of nothingness... and when someone is finally "picked" it feels like artificial development, which tends to be VERY predictable as well. Also, they are cheap and easy drama fuel, it's a cliche I stumble into so much since I gave a shot to the romance genre in general, it is ALL over the place, I am so tired of it that it irritates me.