She’s a champion (aka final fucking boss) but she’s not lame like some are. Add in the sex appeal thirsty fans will give her, it’s a recipe for success. Look at the champs, it’s obvious why they are/arent popular:
Blue is Gary motherfucking Oak.
Lance is cool but doesnt have waifu power.
Steven is fine but not memorable in the original games, and Wallace is a faggot.
Cynthia is cool and has some scenes to make you remember her like Lance did and is a pretty woman and as such has waifu appeal. Being the last boss, all of that’s a shoe-in for popularity.
Lion mane unova man is whatever. Not very cool. He’s there?
Fairy lady XY is whatever. Not very cool. She likes to play dress up maybe? Dumb clothes and hair lower the waifu factor. She’s just not cool.
I didnt play the other games.
tl;dr she’s basically rule 63 lance of course she’s popular