Personality extends beyond just what a person is like, you can see it in how a character moves, attacks, reacts, etc. It's a pretty important thing for a Smash character to have as it also helps make them more fun to play, and in this regard the Mario cast is extremely rich, you could continue adding new Mario characters from now and until the end of the next decade and they'd still bring something unique to the cast.
Actual deep personality traits will never matter for Smash as we won't get to see them anyways, characters which have one strong defining trait that's at the absolute forefront will always shine much more, even if they're one dimensional in their own series. Some of the characters which show the most personality in Smash are the likes of Kirby, Dedede, Jigglypuff, Luigi, Wario and Donkey Kong, not exactly great examples of deep characters in their own right.