Perhaps the most baffling part of Isaac's popularity to me is how many non-fans will claim his popularity is practically nonexistent. I never saw this kind of thing nearly as much with other fan faves like Ridley, KRool, Geno, Banjo, etc. That is, where people would straight-up deny their popularity. There seems to be some weird effect going on where certain circles just don't pay any attention to Isaac for whatever reason, no matter how big he is everywhere else.
There's only like one other character I can think of who got something close to this, and it was Chrom.
Back during Smash 4, a
lot of people tried to write how present he was in speculation off as "he's not really
wanted, just expected" and calling him the flavor of the month pick. A lot of people were either in full-on denial or genuinely couldn't grasp that, yeah, he
was wanted, and not just expected. And that popularity never really went away, even into the ballot era, which saved him come Ultimate. And his popularity in the Fire Emblem fanbase itself also never went away. But regardless, even today you'll still see the odd, and thankfully far less common, person claiming he's unpopular, though they're quickly shut down.
It's not an exact match for the myopia that often plagues Isaac, but it's probably the closest I can think of. And part of me
has to wonder if it's at least partially based in anti-anime swordsmen sentiment.
Besides Chrom the closest one I can think of is the sect of people who write Waluigi off as a "meme pick" but those are usually very obvious trolls.