I didn't realize it went beyond the Arcane fans but I'm not surprised honestly. People are pretty shitty online over the smallest things that don't concern them in the slightest bit.
I think a lot of the genuine nature of it is from the fact M is just a creative person in general. There were several of their streams where they just talked to chat and worked on wigs or props and you could just tell that they were super into it. Plus they didn't try and feed their ego or anything either and if they showed off elaborate pieces, its because they were proud of them. They're just a chill, nonbinary gremlin that enjoys making costumes and dressing up.
As far as cosplayers similar to M, these are the ones I could think of off the top of my head that match them in terms of general vibes and enjoyment of cosplay and/or aesthetics: glitchy_rogue, ann_mossy, odfel, sinty_cosplay, tinkeringpixie, valiantf0x, minakess, olivecosplays, luxstrous, ryesugar, martycipher, and savannah_solo.