This page explains in detail the interactions between The World God Only Knows series and the Mario series.
The World God Only Knows OVA Reunion[]
October 16, 2012
Mario
The World God Only Knows
The second OVA in the series is about the start of Summer vacation, so Keima plans of playing videogames all summer. Near the beginning of the movie, he's seen joyfully jumping and then he's shown in 8-bit graphics, wearing Mario's clothes and in an enviroment based on Super Mario Bros. while performing the infinite 1-Up trick on a Koopa Shell, as a timergoes up to 1000, representing the 1000 hours he plans on spending to play games.
License[]
The creators of The World God Only Knows held no right over Nintendo's Mario, but they only included a minor reference to the game.