When it comes to the "viability" of Ashley's place as a playable character in Smash I think a few things have to be considered here. No matter the amount of games its had, the WarioWare series has never really been considered as one of Nintendo's big, landmark franchises. Even Star Fox and F-Zero have that over it, considering their age (and I laugh at this idea that Falco and Wolf are so obviously on a lower rung than Ashley). It's a small, mostly handheld-focused series that focuses on a series of fun minigames, and when you add up the appeal of the series, its characters fall far behind the microgames themselves. When people say she's a "side character from a c-tier series", what they're really saying is she's one of many characters in the series whose "stories" are only several second long bookends that serve as a pretense for their microgames. Obviously a series' popularity isn't the only basis that Sakurai uses to decide which characters to put in (yes, I know about Ice Climbers), but it might influence where he looks. It makes a lot of sense to me that he would look at the WarioWare roster and go "those would be some great assist trophies!" over considering them for characters, no matter how much I like those characters.
All that's not really that important in comparison to the fact that not only is Wario Ware not a huge series in the scope of Nintendo's universe, but that Ashley is one of a large ensemble of characters. I'm struggling to see what brings Ashley so far above her fellow WarioWare characters besides a cool design and some Internet fandom. WarioWare features a huge cast of characters whose personalities are very distinct and expressive, but are only featured for seconds at a time. Why Ashley over 9-Volt, Mona, Kat & Ana, Orbulon, Jimmy, etc etc? And when it comes to each of those names, why them over Ashley? I really can't imagine any thought process that would lead Sakurai to pull any one WarioWare character out of the game and say "yes, you will be this series' representation." Even within WarioWare itself Wario's personality overshadows everyone else's pretty significantly.
I could even see him doing that for someone like Samurai Goroh over a character from WarioWare, and I don't think he's got a chance in hell.
As for Ashley being "more important" than Rosalina, come on son. I'm not gonna go all "Rosalina is the greatest character of our age" here, but Super Mario Galaxy makes WarioWare a D-list series. For better or worse, Rosalina was always going to have a much better chance than most other characters out there.