Mewtwo can easily get away with not having its Mega Evolutions in gameplay whatsoever outside of a Final Smash, however. Fairy-typing would obviously be a new aspect etched straight into Jigglypuff in that she is and forever will remain part Fairy-type in Smash Bros. And I don't think I have to justify her not using a move related to Fairy-types yet when Fairy-type didn't even exist in Brawl.
Lucario's very heavily Aura-based like he was in the Lucario and the Mystery of Mew movie. In that movie, he used no Steel-type moves either, almost exclusively relying on sensing presence through aura and spamming Aura Sphere. It's understood that Lucario in Brawl was heavily based off this incarnation and thus Steel-type moves would have no real purpose on Lucario. Jigglypuff isn't constricted in the same sense. Just move Sing to her Neutral Special, Rollout to her Side Special, add a new Fairy-type Up Special, and boom, Jigglypuff stays in character while representing a new part of the Pokemon franchise. Ivysaur, for the record, makes use of unspecified powder moves as an Up Smash, Up Aerial, and Down Aerial. The brightly colored hue could indicate that it's Poison Powder, meaning Poison-type is at least nodded to in Ivysaur's moveset.