To put it simply, Porky basically has to stay the same size as Ness, or else he'd be hilariously too small in this case, but at the same time, to scale Porky to that size, you'd have to compromise. Bowser Jr. works because he's to scale of how big Bowser Jr. should be. Porky has to be to scale with Ness.
When you imagine Porky, you more than likely are thinking of his mech when you actually fight Giygas, which frankly would not work, since to keep Porky up to scale, he'd HAVE to be, by far, the biggest Smash character in the game.
Now, if you shrink the mech to actually look like that sprite, small, dinky legs and all and making it more of a glorified hovering chair, then that'd work, but when people are thinking Porky, I don't believe that's what they want(And even then, Snake's grab will clip through the glass and look hella weird).
And just saying it, his Mother 3 design has no way in hell of being playable, for all the reasons above.