'Rubbadubbadub is that true' originated from the First 'Rythm Heaven Fever' episode when during the Wrestling Interview minigame the reporter asks "Rubbadubbadub is that true?" on which the wrestlers just answears with "Yeah!". This is the same fun little game that spawned the quotes "Wow you go big guy!" and "Pose for the fans!" which are all things that the wrestler is being ask during the interview mini game.
This has been noticed in multiple liveshows and The Chatroom even called "The Next Cool Story Bro"
(aka: another goddamn Rythm Heaven Fever Page)