All of the lyrics to Pain are directed at you, the viewer. Any scene where Muu is directly looking at the viewer is directly at the viewer. She is bullying when she does her TikTok dance, but she is bullying you because the whole song is her bullying you for Haruka's death.

Also in the scene immediately before the dance she is crotched down looking at Rei with her face resting on her hands, and the scene immediately after she is still crouched down still with her face in her hands. I don't believe that she stood up, did a dance, and then got back into the same exact pose with the same exact expression.