In my honest opinion, Ness, Lucas and Porky are the perfect three Mother reps for smash. No more than that.
Porky is a huge common ground for Ness and Lucas since his actions connect the two games, plus he’d be a unique fighter and people won’t complain about all the Mother reps being too similar like what happens with the Fire Emblem cast.
I agree completely about the Ninten stuff. He is unironically too similar to Ness to the point where even having him as an echo would be a little confusing.
Design isn't the only things that go into making a character. Yo-yos weren't even present in Mother so it wouldn't make sense to have those in his moveset. Ninten also doesn't learn PK Starstorm so he'd have a different final smash (it isn't even in the original game), and he's more so a defensive PK user in general.
My problem with Ninten's inclusion is that they'd ignore his possibilities and make him just another Ness clone, like they did Lucas. Lucas doesn't learn any of his specials in Mother 3, and even Ness doesn't learn most of his specials. This trend leads me to believe Ninten would just be watered down to be yet another semi clone for the series, which is something I think unfairly represents such an iconic game franchise.
Masked Man is also a great idea for a rep, with seemingly infinite ideas for movesets. Problem is, like many of you have pointed out, he's only in the last game, so it seems somewhat redundant to include him.
Which is why Porky just feels like the right one for me. He's a recurring villain in the series, and his mech being much larger than Bowser Jr.'s would probably make him a quirky and unique character.