Batman v Superman: Dawn of Justice provided our first look at this Apokoliptic reality, and while Snyder didn’t elaborate on how Joker fits into the picture, he described Batman’s arch-nemesis in this sequence as “sort of the Ghost of Christmas Yet To Come, supplying motivation through terror.”
That being said, Zack Snyder’s Justice League did offer the director to finally write something involving Jared Leto’s Joker, who wasn’t present in Snyder’s original vision for Justice League (Leto was among the many actors who shot new scenes in October 2020).
Looking drastically different from how he was depicted in Suicide Squad, this version of the cackling villain will appear during a new Knightmare sequence, i.e. a look into a future where Darkseid has conquered Earth.