Wow, I'm as indecisive and unpredictable when it comes to opinions as I am in life.
Scrap this post. While I do think Mewtwo is more deserving, I actually do think Jigglypuff should remain in Smash Bros. Ugh.
If I made decisions based on who was deserving
without taking note of their own importance in the Smash series, then yes, Mewtwo is more deserving. Jigglypuff may have lost her importance in her own series, but she's still an important part of the Smash Bros. series. To take her away is like when they took away Mewtwo - unnecessary.
Although Mewtwo is more important, if it means replacing another character, then I'm against him now. If he took a fifth or sixth slot, even at the expense of Victini or Zoroark or Genesect, then I'd be fine with him... I think. My opinion can change rapidly over the course of two or so hours.
So I guess I support Jigglypuff again.
Don't shoot me.