Back when Carey was a mentor in 2008, she was a "nightmare to work with" according to sources on the show. Apparently she was extremely late to the shoot and, a far greater offense, "not very nice at all." (Since when are divas supposed to be nice?)
Perhaps as an attempt to stir up that rivalry with J.Lo, TMZ makes sure to include a mention of what a sweetheart Jennifer Lopez was. In her tenure as judge on American Idol, Lopez was focused and work-oriented — "night and day" compared to Mariah. But hey, maybe Mariah has matured over the past four years. That would just kill some of the fun.