How many times do we have to keep bringing up that the reasons Mai most likely didn't get included have nothing to do with her boobs?
The Japanese ratings board have a habit of turning every female model in a game upside down to check for panty shots. If you recall, Mai wears a form of kunoichi dress that exposes both of her legs almost completely, and with how freeform the dress moves it could easily expose more. Doubling that on top of how inherently lewd she is personality-wise, it's no wonder Sakurai and his team didn't want to mess with that after the headaches they've had with CERO and Palutena.
https://sourcegaming.info/2015/09/18/cero-and-palutenas-censorship/
Shantae wears a form of bedlah attire, which depending on the art style can be slightly see through. She's never had a 3D model before, but considering that her games have never included sexual content or been rated anywhere near Adult in Japan, it's safe to say that Shantae's 2D sprite isn't enough to warrant any problems from CERO. I'd say just make her pants not see through at all and turn off the jiggle physics. There may not be a problem in the first place, and if there is it wouldn't be difficult to fix.