Alright, I have a feeling this is going to get me sporked, but--I really, really don't think the fandom is to blame for Itto not being taken seriously. As tragic as his background really is, let's face it: canon treats him like a personified running gag. Even in his own story quest and Perilous Trail that were the two most serious depictions he was given, he was still characterized extremely goofily. He's almost never portrayed with more serious emotions, which does more harm than good even if I genuinely don't think MHY owes us some serious cutscenes.
However, to play devil's advocate, as both an rper and fanfic writer, it does get a bit frustrating when those niggling doubts crop up that makes me wonder if he's not being OOC times. Then, I quickly remember that showing more serious facets of a character, whose background could put him up there with the broody Genshin crowd, is something you just have to figure out. Lord knows I do try my best, and if people take issue? Well, too bad, but with nothing to go off of, you literally have to build the plane as you're flying it, so to speak.