The source artist kinda forgot to pay attention during the Shinsou fight: MHA explicitly says that the school's entrance exam is flawed, so Shinsou fell through the cracks despite having a Quirk that cleanly defuses crimes.

Mineta gummed up the joints of the robots, rendering them immobile and getting him points. Shinsou's Quirk was entirely useless for the exam, since he can't brainwash robots and brainwashing another applicant would be fruitless as they would still get the points.