Shit idk, maybe there's people that don't like edgelords?
Also as for your definition i'm sure you're not wrong but words change over time, considering the majority of people call characters like him, shadow the hedgehog, Reaper, etc edgy characters can follow and edgy theme and their definition is fine.
Why the boards don't love him idk, i guess his appearance is just too edgy compared to jhin who just acts like he's an edgelord on drugs. However as jhin words his BS in such a way it sounds philosophical people would love to get on their knees for him. He does have an overloaded kit tho.
And i mean his appearance srsly pushes me away from him, the whole discount sasuke animu edgelord thing kinda wore off on me. I DO like Reaper in overwatch for example, not like my favourite character but i do like his for of Edgy. And even going back to the anime example Sasuke is just an edgy lil bitch while itachi is certainly edgy but still cool.
Also saying 80% of the champs are anime thropes is a bit much. I think we're past the point where 80% here have long emo hair. The ones that are fine with kayn don't care, those who have complaints make themselves heard.
You can be shooting rainbows ingame, if your lore is edgy you are edgy. I your looks are this edgy tho, your lore won't even matter.
tldr
Edge isn't bad if handled correctly and even then it's personal preference. Vocal minority, words' definitions change over time, lore/looks is/are often what defines a character.