It all started when the British pop innovator rocked a sash that said, “Miss Should Be Headliner,” at her afterparty on April 12 following her Y2K-fueled performance at Coachella Weekend One.

After Charli’s performance at the festival’s opening weekend, she was spotted at an after-party wearing a sash that read, “Miss Should Be Headliner.” Very, as the Brits would say ...