In-game, Koopa shells can be ridden after being detached from the enemy's body. Without its shell, the Koopa will be shown in its underpants, and run around embarrassed. Shell-surfing protects Mario from several hazards such as lava burns and damage from weak enemies. The shell will break if it hits a wall, however.
In OnyxKing's bloopers, Koopas are minor characters that rarely appear. They are said to be very retarded and the like to call people fat. Their in-game behaviors are also used for comedy.