@OP Mario has significantly higher Mobility than Luigi or Doc, which is extremely useful. Mario is much faster relative to the rest of the cast than he was in previous games. As a result he has more effective ranges than the other versions. That is probably the biggest thing he brings to the table.
Also Ally is a genius, seriously. He is perhaps the best smash player in the world. The reasons he does things, is because he knows they will work. If you can read your opponent, and are capable of punishing (any character with reasonable mobility can do this) then you can beat anyone. Mario has plenty of solid stuff. Granted hes not as good as say Luigi, but Mario has tools to accomplish basic things you need to win.