Why does everyone hate Otoha here?
For her toxic relationship with Rin?
Because she's manipulative, selfish, hypocritical. Uses people and lies to seem more interesting. Not smart enough for her to be particularly intriguing, not dumb enough for it to but cute. And more thoughtless than malicious overall.
In fact, she occasionally realizes how shitty she's being, and wants to be better but either has no idea how, lacks the motivation to try, or is too stubborn.
In short, she's the most realistically written teenage girl, and that makes her frustrating.