In order to trick the Batman Who Laughs, Superman decided to go undercover. If he really did infect himself with the Joker venom to turn into the Superman Who Laughs, I don’t know. Seems risky though if he did. Maybe a lesser or weaker dose of the venom so he can still maintain his mental faculties? I doubt Batman would go along with a full dose. Or maybe Clark’s just acting with some really good makeup on.
From – Batman / Superman Vol. 2 #2