The only point in time in which a mascot was close to outselling mario was in the PS1 era, and that was crash bandicoot, we can see where he is now.
And no, no single character in gaming can become bigger than Mario.
Bet with Dr.A.Peter.Nintendo that Super Mario Galaxy 2 won't sell 15 million copies up to six months after it's release, the winner will get Avatar control for a week and signature control for a month.