Warning: contains spoilers for Batman/Fortnite: Zero Point #2!
While Batman and Catwoman basically do everything but what's expected of them while trapped inside Fortnite, Harley Quinn is currently slaying and emoting hardcore, to the point where Batman has actually articulated concern for her well being. It's been clear since the first issue of Batman/Fortnite: Zero Point that Harley wanted in, jumping gleefully through the tear in reality that appeated in Gotham City. Since then, she's fully immersed herself in the game.
Meanwhile, Batman and Catwoman have been trying to break free, but their efforts are greatly hindered because they can't talk, have no knowledge of who they are and have all of their memories wiped at the end of each round. Once they realize they're trapped at the start of each new round, the two spend the rest of their time exploring their prison, analyzing clues and plotting their escape. Meanwhile, Harley is battling like the excellent Fortnite player she is.
Tens - or possibly hundreds and thousands - of battles later, Batman is able to recognize Harley in Batman/Fortnite: Zero Point #2 by Donald Mustard and Christos Gage. When he tries to warn her of what he and Catwoman have discovered, Harley frantically fights back and pushes him away, confirming that she wants nothing more than to continue battling in this world. But what proves that Harley has truly embraced their new life is that she now dance emotes wherever and whenever possible. Even Batman realizes this isn't good when he says, "She's embraced the dancing. That's a worrisome sign. But if she's been here alone as long as the Cat and I have been here together... It's no wonder she's gone native."
As he says these words, Harley can be seen performing a dance emote. And as most people who play Fortnite know, dancing is the most satisfying way to express a victory and to rub the noses of one's opponents in their loss in the most humiliating way. Based on her poses, it can be assumed that she has performed this dance numerous times, and is used to seeking out triumph in the game.
It shouldn't come as a surprise that Harley Quinn would become the queen of Fortnite, prone as she is to expressing her destructive tendencies and the joy she finds in violence and mayhem. Although it's unclear whether Harley and Batman were allies before the Fortnite debacle began - as they currently are in DC's main timeline - Harley Quinn is still known to exercise what she has called "ultraviolence" during her reformed days, so it's still within her character to enjoy meaningless violence even if she's no longer a villain. In fact, blame-free violence where all the victims are consenting and return to life once the round is over may make Fortnite close to Harley's heaven.