I mean, the answer certainly can be different, but Ahri is genuinely popular in the fanbase. You don't get the amount of skins she does if you're not popular.
That said, she's far from the only option. Most people have already called a few names, but Katarina, Jinx, Vi, Miss Fortune, Teemo, Yasuo, Lux, Ryze, Ekko and others are also popular characters that could rep the game well enough.
To paraphrase the LoL general thread: League of Legends is one of the most successful MOBAs out there, with over 100 million users (
data from 2017) and is one of the biggest Esports
period. Riot Games is also owned by Tencent, who has approached Nintendo for businessin recent years - notably with Pokémon Unite, Nintendo's own MOBA - and also helping to bring the Switc to China.
LoL is also getting stuff on the Switch - Ruined King is confirmed and Wild Rift is pretty much a matter of when. So there is some Nintendo connection.