I'd say it is irrational, considering that most folk that hate it have never actually listened to more than one or two Metalcore bands.
So how many would you say I have to listen to before I'm not deemed irrational that I hate "metalcore"?
Why is Metalcore in quotation marks? Are you trying to imply it isn't a real thing?
Maybe irrational hate is the wrong term.
What I find irrational is people who listen to a couple bands they don't like and decide that the entire genre is the drizzling shits. If a person has actually listened to a fair bit of this or any genre for that matter, and still dislike it, fair enough. But to say that all of a genre is shit because of a couple bands you don't like is idiotic and, yes, irrational.
I've heard some "metalcore", and I can objectively say that it all sounds fucking horrible, or as you so aptly put it, the drizzling shits.
Why do you keep putting it in quotes? Seriously, why?
Music can't really be objectively bad, though, because there's always gonna be someone that enjoys it. You can think it's bad but that doesn't make it objectively bad. Do you really think that much of yourself that you believe your opinion to be absolute unbreakable fact?
Oh, and out of curiousity, what are the Metalcore bands you've listened to?