pain-into-pleasure
Explanation
The Joke
A man announces that he is wearing a suit that converts pain into pleasure. Superman approaches and punches him, expecting a normal fight, but the man explains that the kinetic energy from Superman's fist will be harvested and used to "gradually stimulate the region where the brain would've felt pain." He further clarifies that any leftover kinetic energy will be converted into a pneumatic nozzle that sprays essential oil of lavender. Superman is baffled and says he doesn't see why this should allow the man into the Justice League. The man's adoptive parents (shown as an ordinary-looking couple) are called "a couple of weirdos."
In the next panel, the man warns Superman that he wants the suit to be used "precisely." Superman asks what that means, and the man simply says "Sure, that's a word." In the final panel, set "later," we see someone yelling "Stand down, Luther! Or I will throw Captain Titillation at you!" -- revealing that the man has indeed joined the Justice League under the name "Captain Titillation," and his power is essentially being used as a human projectile whose pleasure-suit makes him an uncomfortable weapon to deal with.
The Humor
The comic plays on the absurdity of superhero origin stories and the increasingly bizarre powers that comic book characters possess. Instead of a conventional superpower like strength or flight, this character has a suit that converts pain into pleasure -- which is less of a combat advantage and more of a fetish. The humor escalates as the Justice League apparently accepts this ridiculous hero, and the punchline of "Captain Titillation" being thrown at Lex Luthor as a weapon perfectly captures the absurdity.
The joke also pokes fun at the arms-race of weird superhero concepts, where the bar for "useful power" gets lower and lower. The detail about lavender essential oil spray adds a layer of comedy by making the suit sound more like a luxury spa experience than battle armor.