Almost every popular character in one piece has a toxic fanbase, and those who very popular has the worst base.
As a Luffy fan I have to say his fandom can sometimes be a bit toxic but I truely never seen it go far and beyond stupiditly annoying as other popular character's fandom do and anyone who argue about this is either a hater or not telling the truth cuz I've been here since the start even before I joined and Luffy fans are mostly respectful and only here to discuss for the sake of having a good productive conversation to enjoy the manga they love away from intentional trolling for the sake of annoying others and having a laugh.
Now, lets speak about the most toxic fandom I've seen and they're in no doubt Zoro fans. Seriously, I love Zoro and he is my second favorite character and no toxic fandom will change the fact he is a very well written character that deserves the admiration he gets but his fans only try to troll others to annoy them and to force others to accept things that aren't true about Zoro. Also they have this tendency to falsely claim things to hype Zoro or dehype others. Twisting facts and bringing headcanon false claims and speak about it as if its a fact.
Zoro fans remind me a lot like Hunter x Hunter fans, both go extreme in their praising of their passionate character or show and in the same time disrespecting other good shows and characters to make themselves feel good and superior over others.