Classic Prank, But There's A Gaping Plot Hole...
Why is the Joker in the Wayne Manor? Why is Batman still wearing his mask in the shower? In fact, why is he even parading as his superhero persona in this supposed Wayne Manor? Is this even Wayne Manor? Maybe this is not Wayne Manor, maybe this is some other person's home, like Vicki Vale's or someplace, and Batman is just here for that person's protection, but he needed a shower because he smelled real bad, so he decided to take one for the request of the housemate, but the Joker managed to sneak into the household, and snuck into the bathroom just to pull that prank before causing more trouble like holding the housemate hostage. Or Maybe, the supposed person who Batman is supposed to protect in the house IS the Joker, because the Joker is really good at disguising himself, and The Joker actually arranged this just so he could pull that prank. Or maybe, this is just another random DC "elseworld" where the Joker and Batman are a gay couple.