Seriously, why would we have a third mother character if it wasn't Ninten? 3 characters to represent 3 games. That's the only justifiable reason for Mother to surpass 2 representatives.
DLC would make it justifiable. The Mother fanbase would more gladly pay for another character, more so than most other characters' fanbases, considering how devoted the fanbase is. Since all the possible choices for another EarthBound representative comes off as easy C-list choices compared to the likes of Little Mac and Ridley, the likes of Porky or Masked Man being added in would be based off of demand rather than "representation". That said, I don't think Ninten overshadows either of the Mother antagonists for support. Hell, I'm pretty sure
there's currently more Jeff and Poo support than Ninten. In a DLC setting, the money-maker would be the one added, which is why I have my doubts for Ninten being DLC for that purpose. Meanwhile, Porky has familiarity from Brawl, a play-style that would be more niche to the EarthBound cast in Smash Bros., and is easily a fan-favorite to the series as a whole. When expanding this all out to an audience that may know nothing about EarthBound at all, between "Ness's little brother that can shoot beams" or "The pig-like kid who fights with dirty tricks", who do you think will sound like the more enticing choice as a brand new character?
And still going for Ninten, he represents Mother less than Porky, ironically enough considering that's the reason you support him. Ninten would represent "Mother series protagonist" and "EarthBound Zero representative". Porky would represent "Mother series antagonist", "Non-PSI user", and "Recurrent character" all at once. Ninten represents one game of the series, but Porky represents an entire chunk of the series as a whole. Ninten's only promotion potential is for the first entry in the series, an NES title that was last acknowledged by Nintendo in 2003 in the Mother 1+2 port and hasn't been mentioned since. That type of niche would work if we were passing off Ninten as a retro-rep, but then he immediately faces stiff competition in Takamaru, Lip, and Mach Rider. Porky on the other hand promotes both EarthBound on VC and support for a localized Mother 3. Objectively speaking, Porky is the better choice.
All this is assuming the vocal majority wanting Waddle Dee and Krystal don't get in the way of a DLC Mother character in the first place, considering either of those have much more sales potential than either Ninten or Porky.