It's important to note that Ashley being overwhelmingly more popular then everyone else is only a Japan thing, some evidence for that has been pointed out here :
http://warioforums.com/index.php?threads/whos-actually-the-most-popular-warioware-character.1202/
Sure, she is still popular, but not to a overwhelming degree and it could even be argued that 9-Volt beats her in that regard, which is supported by the fact that a good number of Ashley-only merch (cleaning cloths, badges and the second 3DS theme in Europes's case ) never made it outside of Japan (as of now, anyway).
The fact that the tropes that she embodys are particulary huge in Japan is also notable, which makes it easy to slap her mug on something and expect even people that have no idea what WarioWare is to buy it, similary to how people buy easy to market stuff like Hello Kitty merch, despite not knowing where that thing even came from.
I also have to question how she is ''unique'' in comparison to other Nintendo characters, given that most of her supposed personality traits are hardly (if ever) touched upon and she never did anything oustanding that would make her stand out, unlike someone like Wario, I'd even say that 9-Volt is more unique, due to his very meta nature of being a Nintendo character, that is a Nintendo fanboy and even then I still wouldn't approve of him being put above the other characters, they are best served when they are all together on the same level as far as I'm concerned.