First, when you fight him for the first time, you see a 5-or-so-second montage of his fist meeting every other boxers' faces. Then, if you have the presence of mind to look, you'll notice he has zero losses on record.
When he comes back for a rematch in Title Defence Mode, he gets a new montage. He punches the walls of a brick building, and then the whole building collapses. He punched out a building. Dude is scary.
The fact that he was the former champ at all is a moment of awesome. Think about all the other boxers in the world circuit or hell, the entire game. All those boxers got where they are by either cheating, or using a gimmick to win for example, Soda Popinski drinking soda to keep his stamina up or Aran Ryan using underhanded tactics to get a punch in. How did Mr. Sandman get all the way to the top then? By just being a good boxer with powerful punches.
Beating Mr. Sandman in Title Defense Mode on the Wii. After he takes enough of a beating, he'll start swinging too fast to counter. If you can dodge long enough, he'll completely drop his guard out of exhaustion. This enables you to combo him as much as you want, and get stars for doing so. Finish up the combo with a 3-Star Punch and you win. Awesomely.
There's another one for the fight against Mr. Sandman, but for Little Mac instead. When your first fight with him starts, it shows him entering the stadium, so large that the ground is shaking with every step he takes. When Doc sees him, he lets out a small groan of fear, followed by another more questionable one at Little Mac as if to say, "Are you sure about this?". Little Mac's response?... He furrows his eyebrows at him, pats his fists together, and then stands up to start the fight. Considering the fact that he's doing this against a man who is twice as tall as him, over a hundred pounds heavier than him, and is notorious for knocking all of his opponents out in one punch, only further proves how brave and determined Little Mac really is.
In Title Defense, Mr. Sandman's re-introduction consists of him taking a gander at Little Mac and immediately goes into a roaring rage. You know you're one of the best boxers in the association when you turn a previously-undefeated boxer roughly twice your size into a raging Sore Loser.
Doc Louis gets one with five words.
"I invented the Star Punch!"
The voice acting, and the fact that every boxer speaks their native language.