Well, to be fair, even though Ashley didn't appear in any GBA WarioWare game, it is a very popular character among the fans, pretty much almost (if not as) as popular as Mona, which is WarioWare's second most popular character, being Wario - of course, the first. Considering it was created in the fourth installment of the series, and only appeared in 1/3 of all WarioWare games, it's quite a stunt to become this popular this quick, IMO.
Edit: Oops, forgot about WarioWare: Myself. It'd be 1/2 of all WarioWare games, sorry!
Edit: Oops, forgot about WarioWare: Myself. It'd be 1/2 of all WarioWare games, sorry!