Jennifer Lopez threw a ton of shade at Mariah Carey ... by burying her head in her smartphone while MC was hitting high notes onstage at the Billboard Music Awards -- and the video is epic. <br />You've gotta see it -- while Mariah was singing "Infinity" ... J. Lo was casually scrolling through her phone, barely even glancing at the stage. <br />Even as Taylor Swift and the rest of the audience burst into applause for Mimi ... J.Lo chatted up Casper Smart from their front row seats. Jennifer's no fool though -- when the house lights went up at the end of Mimi's act, then and only then did she give her a hand. <br />It's hysterical -- you get the feeling Mariah could have been on fire and J.Lo wouldn't give a crap.
