Although relatively unknown, this game has the distinction of not only being the first Punch-Out game, but also the first ever cameo appearance of DK, DKjr, Luigi, or Mario (Donkey Kong Jr. aside). Nope, Mario isn't the ref in this game. See for yourselves ... Notice anything? Look to the left of the ring. All four of them are sitting in the audience. Here's an enlarged image.
Mario is in the lower left corner, above the announcer. DK is sitting just to the left of the ring. DKjr and Luigi are a row behind them (Luigi can just barely be seen next to DKjr). Donkey Kong will cheer for the opponents. The four can be seen on both sides of the ring ala Kirby Super Star. Thanks to the DK Megasite for this information.
Unlike the SNES game of the same name, there are no significant differences between Super Punch-Out and Punch-Out. In fact, the two games were released within just a year of eachother. The only differences are that there are new opponents, and that the colors of the ring and crowd were changed (the exact same crowd sprites were used). Nintendo's four earliest stars appear in the same places as they did in Punch-Out!!