When it comes to the WarioWare originals, Ashley absolutely outshines all the others in popularity, to the point where she got to be designed in a ton of various outfits, including in other Nintendo characters outfits, and she even had her own dedicated April Fools event. She also got to be in Smash Bros with a 3D appearance, the only others being Kat & Ana, she's the only WarioWare character with a Mii costume and her main song from WarioWare Touched got remixed twice, with one in Brawl and the other from Smash 4. And out of all the WarioWare characters, people who want a WarioWare rep to be playable in Smash have been the most vocal with her, since she has a lot to offer in terms of moveset potential.
Oh, I remember... ngl, this huge focus on Ashley during a time where WarioWare's future was in question sort of bugged me more than you'd think...
Mona has been in the series since the beginning, although she didn't appear to get a push in popularity until more recent games with WarioWare Gold and Get it Together. So in a sense, Ashley did upstage her in popularity kinda like Morrigan with Demitri(Also ironic how Ashley was given the cosplays when Mona's the one into fashion. Mona cosplaying as Nintendo characters would certainly give her a boost). Today, Mona can be considered the character who gets pushed the 2nd most by fans, especially in regards to an appearance in Mario Kart where she can fit very easily. She still has nowhere near a push into Smash Bros compared to Ashley, but Get it Together did give her an overall decent push in her favor.
As someone whose favorite WarioWare character IS Mona, I feel like Nintendo hasn't been using her to her full 'potential'. Like you said, as someone into fashion, it's pretty surprising that Nintendo hasn't played with the idea of Mona cosplaying as other Nintendo characters.
Also, I feel like the "weird" aspect of her character is suffering from a case of "tell, don't show". I've seen folks who consider Mona pretty bland because she doesn't seem to have much in the way of a stand-out personality. Sadly, I sort of see it. Like, the interesting stuff about her is only ever alluded to in text. I feel like Nintendo should lean into her odder tendencies more. Maybe play up her obsession with Wario (in a non-creepy way, if possible), perhaps show that her various odd jobs are taking even more of a mental toll on her than she herself lets on, etc.
I remember someone who drew this Mona fanart on Twitter sometime last year that I think REALLY encapsulated how Nintendo should have handled Mona ever since Gold, if not since the very beginning (in Mega Microgame$). If only I could find it...