>>41388360Xenophilia and social class stuff like
>>41381452 said. It's got overlap but it's different from loving a Mutant like Emma Frost or Jean Grey because Gardevoir occupies a distinctly different species and social class than you do. She's not just a cute girl with superpowers, she's a cute girl that looks like a bugeyed alien that people can catch in a ball and force to do their bidding. You know how some media like Anime will have elves or demi-humans occupy a subservient slave class in society? It's like that.
Plus there's the thing where her dex entries say she's undyingly devoted to her trainer, and it's nice to imagine somebody who will love you no matter what. There's instances of this "life bonding" idea in a lot of other sci-fi and fantasy works too, so it's not uncommon.
Finally she's just attached to a big media property, so unlike those fantasy books she's easy to get attached to in cartoons and games, and then carry that attachment into infatuation. And since there's a lot of other people that feel the same, there's a communal feedback loop that leads to more people becoming attached to her.
She's just a very visible character that hits a lot of different checkboxes for a lot of different people.
