>>149296980
Shigaraki tactically used Lemillion's provocation to fully awaken and channel Tenko Shimura inside of him so, with the full influence of his Tenko persona, he would be able to tip the scales of his body in his favor and cast out All For One from himself.
What might seem like a retarded childish outburst to a simpleton or a speedreader was in actuality a brilliant ploy to gain full control over his own body while also taking AFO's power for his own. It was also made Lemillion look like a retard since his big "you're bad cause you have no friends" gotcha got destroyed, but that's besides the point.