Diddy is probably not that popular because he's a sidekick. People would prefer choosing the heroes or villains than the sidekicks... Maybe thats why people use the more popular "starring characters" and the "arch nemesis", rather than other secondary characters. Diddy is a sidekick, and doesn't appear in many DK games. DK is the starring character in DK games, so people would feel compelled to play as him rather than someone who barely appears, or who doesn't leave a lasting mark on the player (unlike Sheik, for example).