I'd like to point out, that you are wrong.
Jigglypuff has no shot? Are you kidding me? Jigglypuff has just as good a shot as Mewtwo. Do you notice something that all these Pokemon characters in Smash 4 have in common? No?
They're all relevant to the 6th generation.
Pikachu is the mascot, and the least relevant, sure, but it is the first Pokemon in game that is actually using voice clips for its cry, instead of electronic gibberish.
Lucario played a small but important role in X and Y and teaches the player the concept of Mega-Evolution.
Charizard is a Kanto starter, which is available in X and Y and you can get early, and has not one, but TWO Mega-Evolutions.
Greninja, is the most popular Kalos starter, really nothing more to it.
So what's missing? What's the one thing that was promoted to hell and back and is yet still not playable in Smash?
Fairy typing.
And who was one of the first Pokemon to be confirmed with it?
Jigglypuff.
I rest my case. Mewtwo has just as good a shot as Jigglypuff. To say Jigglypuff has no chance is horse ****.
Lucario's role is not small. There's a giant statue of him for one thing. Also, Jigglypuff is not even the poster child for the Fairy type. It doesn't even a primary fairy type! It's primarily Normal, secondarily Fairy. I think Sylveon would be a better choice if we're all about Fairy types.
Regardless, I think Fairy stuff is largely irrelevant. We didn't get a Dark or Steel type in Melee. Although it has a bit of weight in favor of Jigglypuff as an excuse to keep it, it wouldn't be a problem if there wasn't a Fairy type choice.
Mewtwo, on the other hand, is actually still extremely important to 6th gen. He's the ulitimate legendary you can catch, the only legendary with mega evolutions, and, with those mega evolutions, the most powerful Pokémon ever with a base stat total of 780, the highest there has ever been. Also, there was that movie less than a year ago.
Jigglypuff isn't special, important, or even popular anymore. If it's in the game, there simply wouldn't be a reason except for that it is a legacy character. Is that important, I suppose so, it hasn't proven to not be yet. We DO, however, only have two examples of legacy characters existing. Not exactly a pattern.
Also, I WANT Jigglypuff in the game. It's my Melee main. I even (no joke) use old Jigglypuff cards as business cards. I write my name and phone number on them and give them out to people at conventions and stuff. I just, unfortunately, don't see it in the cards (no pun intended) this time.
I suppose you're right in it being wrong to say it has NO chance, though. Just a small one.
EDIT: You also say that Mewtwo has no role in XY. Not true, but even if it was, Jigglypuff does? Really?
EDIT 2: You ALSO say that Jigglypuff was the first Pokémon to be announced to have the Fairy typing. Not true either. Sylveon was and even before that, Xerneas, a pure Fairy type, was announced but its type was a secret until Sylveon was announced. Later, Jigglypuff, Marill, and Gardevoir were all concurrently announced to have been retyped.