For me at least it's not about hating her, it's just that outside of being the butt of the fat jokes, she's tremendously boring.
Miku has the same problem. I used to like her a lot at the beginning, but since she went dere, she got reduced to muh Fuutarou and doesn't do anything other than blushing and being jealous. Ichika also got stuck babysitting Miku, losing the edge. Her smug moment during exams was magnificent and I really hope she snaps out of this stagnation and starts competing with sisters, because this whole Miku guilt is getting old.
Anyway, I simply don't think that Itsuki has anything interesting to offer in terms of romantic development, interactions or character dynamics, especially compared to Nino, who continuously manages to keep her relationship with Fuutarou fresh and entertaining. In other words, Itsuki route for me would be boring and a waste of opportunity, so it's simply frustrating when she's in the lead of the meta competition.