You can report comments, and if they are not deleted by the admins it means the comment is not breaking rules.
As a fan of IU, I actually saw that comment and it is not that bad of a comment to justify deleting it. I'd say your response is worst, typing things like, "Why are you here?", "Is your life that bad or do you have nothing useful to do? ", and "I ask you, who gave you the right to be so nasty?". I reported your comment and it was removed.
That user was just saying they don't like her and are questioning why she is popular in South Korea. Even if their take is bad, saying it because of her appearance maybe, it doesn't warrant a comment removal. People saying they don't like something or someone is not "hate comments", they are free to write whatever they want (praise or criticize), just not use slurs or be offensive. Just like the other lovely people who responded to the user, each person has different tastes. So no need to be rude and try to censor people because you can't handle someone criticizing your favorite idol, just block them if you want.
Also, it being an Ultra fan guide, doesn't mean no one can criticize the person written about in the comments. There is no such rule.