Ashley is a weird pick. She's not the Wario Ware number two, Jimmy T. and Mona are the seconds in command, yeah, but Ashley is way more popular than them. Nintendo did some promotional work with her, (badge arcade, April's Fool, mii outfits, spotlights in Ware promos...) so the fanbase grew, Nintendo is using her. She's a character created for beign loved if you ask me. Her situation is almost exceptional.
Wario Ware cast is not a normal cast anyway. No villians, no logic stories, not even heroes. Except the Wario-Jimmy/Mona-Rest hierarchy there's no other thing for logical choices about the series, and that hierarchy is not very strong anyway.
Actually Ashley is over that. It's not exactly as if we got Dixie before Diddy, because nobody is supporting that Diddy, not even Nintendo.
She got fans, potential (even for spinoffs, not only for Smash), promotion and cuteness. Beign a fighter won't be a surprise.