This word about all the DLC characters being selected, and about them being ones that Nintendo picked and Sakurai later approved is interesting. I could go many different ways.
Firstly I think the selections will be more profit based rather than the occasional whimsy Sakurai uses with his selection process. This makes me feel that there could be two directions Nintendo would go: promotion or popularity. Either selecting characters from franchises/games they want to further promote ala Bayo/Corrin, garnering better sales from those games ideally. Or, popularity, wanting to capitalize on DLC sales by selecting characters they know will sell well because of the demand for the character or their overall marketability. (a third motive for Nintendo selecting a character could be for strengthening relationships with a third party, but that seems less likely to me. Any third party included I feel would be because of one of the first two reasons)
I could see this working for or against our girl. Nintendo clearly likes to advertise Shantae (Nintendo Power cover, Shantae Nab, showing up in the in store advertisement for games on the eshop along side Mario and Zelda, and the time she spent as the poster child for the Nindies section of the eshop) So, it's possible they could see sales of her multiple games on their multiple systems increase with more exposure. Or in a dream reality, they could be funding a new Shantae game to be Switch exclusive and include her in Smash in a celebration of the partnership similar to what they did with Bayonetta.
If popularity is their main driving factor, then where they were looking to determine popularity is important. She sells well on their systems, so they know a good amount of Nintendo gamers like her, but just by sales it could still be hard for her to outshine the popularity of well known characters from bigger developers. However, if they are looking to the fans to help determin which characters would sell well, Then Shantae could still be high in the ranks as most all the heavily demanded characters above her have already been made fighters or assist trophies (Tho i will say that now that I know it was Nintendo making the suggestions, i do think for the first time that Assist Trophies could be upgraded if they felt the demand was high enough)
Another thing to consider tho, is that if Nintendo was looking to fully maximize profits of the DLC (promotion for other games and third party relations be damned) it could be possible that all of the 5 will be first or second party properties as that way they wouldn't need to share the profits. I don't think that's incredibly likely, but Nintendo is a corporation that has to answer to share holders and telling them 100% sounds a lot better than 80%, 50% or whatever the deal would be for third parties.