WARNING: The following contains spoilers for Superman: Villains #1 by Brian Michael Bendis, Matt Fraction, Jody Houser, Michael Gaydos, riley Rossmo, Scott Godlewski, Bryan Hitch, Cully Hamner, Steve Lieber, and Jim Mahfood, on sale now
The Joker has a long history of openly mocking the other villains of the DC Universe, befitting his role as the Clown Prince of Crime. He's long mocked other characters whenever they present the chance for mockery, no matter how dangerous they can be.
The most recent case occurred in Superman: Villains #1 when the assorted bad guys of the DC Universe got the chance to respond to Superman revealing his secret identity as Clark Kent. It turns out that when the Joker found out the truth, he went out of his way to ridicule Lex in the pettiest -- and most hilarious -- way possible, expertly trolling his fellow villain.
After the announcement that Clark Kent is Superman, Lex can be found in his office going through some of his work when he realizes there have been ninety-nine different voice messages left for him. But instead of being all the different people of the world trying to reach out to him, it turns out that seemingly all ninety-nine messages were left by a single person: the Joker. Each message is just a series of his trademark cackling and laughter. Lex tries to keep his calm amidst all the giggles, but he barely makes it through the fourth message before he can't stop himself from smashing his devices to pieces and tearing up the Daily Planet that included the story about Superman.
It's such a petty waste of time but it's perfect for someone as needlessly cruel as Joker. The villain had to sit at a phone and leave ninety-nine different voice messages, all just making fun of Lex. That must have wasted so much time that he could have dedicated to his plans for the upcoming "Joker War". But instead, Joker wanted to take the time out of his day to make sure to properly mock Luthor over the fact that he never saw his greatest enemy standing right in front of him. Considering that Joker has been hinted to also know Batman's secret identity, it's especially funny that he wants to rub Luthor's face in it in such a simple way.
Luthor has long been established as someone who can take a simple insult and turn it into an incredible grudge. His hatred for Superman can be broken down to petty jealousy and anger over how he's Metropolis' favorite son instead of Lex. He can't stand being mocked, and this is just outright laughter at his expense. The only words the Joker can choke out amidst his laughter is a feeble "the whole time, Lexy," showing even more disrespect for Lex despite all of his recent shows of power in Justice League. But the Joker has always been skilled at getting under people's skins, as shown here. Coupled with his recent interactions with Lex in Hell Arisen, Joker looks like he may understand Luthor even better than he does Batman.