Nearly four months into her tenure as CEO of X—the “everything app” that has to keep reminding users that it used to be a social network called Twitter—Linda Yaccarino can’t seem to fully explain what it is she’s doing there. This week, the former NBCUniversal advertising chief made the rounds in a sphere where, one would assume, she’s long been comfortable: the mainstream media. On Wednesday, the Financial Times published a first-of-its-kind profile of Yaccarino’s X tenure, covering her working relationship with the ever-controversial owner who appointed her as CEO, Elon Musk. That same day, she appeared at Vox Media’s...