Given how many people claim to love Corrin OUTSIDE of Fates, I'd doubt that claim. Imo it's just that Fates plot betrayed Corrin a lot of times

Maybe, but even then, that's the kind of perspective on Corrin that bothers me the most.

Of course it's not bad to like Corrin outside of Fates, but most of the time their personality is piecemeal and sometimes flanderized. They're not center stage, they're plucked from a point in Fates' storyline (pre choice for Warriors, post choice for Engage, whatever the new alt demands for Heroes) that's intentionally left vague and open-ended, and in doing so, Corrin becomes palatable to more people.They're open minded, sweet, supportive, and that's it. There's nothing that's seemingly "offensive" to people's story experience as usually associated with Corrin when consuming non-Fates material. People can like Corrin without being reminded of why they don't like Corrin. And imo that's a little, uh, weird, I guess.

And I wouldn't have these mixed feelings about it if people didn't claim that Corrin is better written in non-Fates material, but that's often the case, and that causes me to take said opinion less seriously. Cuz at that point, what's actually being liked, is it Corrin, or just the general Protag Personality tropes?

As an example, for as much as I adore Engage, all of the female emblems personalities blend in very easily with each other, Celica, Eirika, and Corrin most of all, cuz they're all hit with the "fanservice flanderization" so they don't the boat with any controversial personality tropes. Yeah it makes sense since they already developed through their games' events and thus don't need to do anything akin to their mistakes in their home games, but when someone says they like Corrin in Engage but not in Fates, what's exactly so likeable about her that I wouldn't mistake her for Celica or Eirika? Or even Marth to an extent? It's the same case in Warriors because Celica, Caeda, Marth, and Lianna are all there too, with similar personality tropes. Heroes sort of gets away with it because "hee hee hoo hoo fun and cute designs" that no one else has and variants that deal with actual character development stuff (Legendaries, Fallens, Braves, Ascendeds) all of which help not just Corrin, but Eirika, Celica and others separate themselves and be true to their home game, wrapped in a hellish gambling scheme.

To cut my ramblings short, I can get why people like Corrin outside of Fates, and I even appreciate to a certain extent, but at the same time it can read as superficial and dismissive to their overall character.