Depends on how we're defining "mainstream audience". There's obviously levels to this kind of thing, as there are with niche products. Nier Automata has more sales than Persona 5 at this point (4 million and 3.2 million respectively as of last year), which puts them above the level of niche cult following imo, especially when both games have had a semi-mainstream stage at the game awards. Shantae on the other hand has no public numbers to my knowledge beyond the first game, which was a massive sub-30k flop iirc.
For Shantae to be more recognizable, we'd have to assume the people that haven't played any of her games are just tangentially aware of her through cultural osmosis, as sales is clearly not in her favor. The problem here is 2B has the exact same thing going for her. To outsiders, she may be known as "the girl with the fully-rendered butthole" (even if they never learned that was fake), but that's still awareness. That's a very clear 4 million+ while Shantae is coming at this from a much lower number+. Let's say for arguments sake the Shantae franchise has sold 1 million copies. That means there needs to be a whopping 4 times the amount of people that have consumed no content with her in it but are in some way aware of her to reach the amount of people that have flat out consumed the content 2B is featured prominently in.
There's obviously no way to get exact numbers on something as hidden as awareness, but we can extrapolate from what observable metrics we do have, and they are not in Shantae's favor.
