The main man of video gaming fame, Mr. Mario himself, is not shy to show off his butt. Here you can see the nice pockets on his jeans as he climbs up the screen in Super Mario World.
His clothes for Super Mario 64 do not quite show off his pockets, perhaps he decided to go for the full body leotard for this game. It would certainly explain how he was able to perform all those acrobatic moves (backflips, triple jumps, etc.)
Crouching down, Mario sticks his butt out a bit.
Crawling is another position that shows off Mr. Mario's backside.