How will the flurry of at-home and in-game performances leave their mark on pop music? “We just wanted to fuck around and have a good time,” Post Malone said, sipping from a beer can in a koozie before holding it aloft. “Cheers, fuckers!” In a thunderous 80-minute set of Nirvana covers, livestreamed from his Utah home on April 24, he was funny and loose, wearing a floral day-dress (not unlike the one Kurt Cobain once wore) and checking if his accompanying band was “hot and ready, Lil Caesar’s style.” “Let’s kick this shit in the ass,” he said, referring to COVID-19, before launching into the whiplashing riff of “Drain You.” Post Malone styles himself as a slouchier version of a golden age axe-wielder. He’s covered rock classics and literally has a Nirvana face tattoo, but on the livestream...