GamePro have shed some light on why..

GamePro: Last question: Why can not Mario umpire?

Tanabe: Miyamoto wouldn't allow it. No just kidding, actually there's a much more practical reason for it. For exampre he just doesn't fit the graphical style of the remake. To make him look good in the ring, we would have to adjust his physical measurements. But then he wouldn't be Mario anymore.

Thats a really good point. A tall Mario would look pretty bad.

Did he appear in super punch out?

"Did he appear in super punch out? "

He's not in Super Punch-Out!! as far as I've played (only completed the first league, 2nd dude in the next one owns me).