Out of all the villains within DC's library, the Joker is one of the rare few who can fit as anyone's enemy — including the Flash's. While he is primarily Batman's rogue, the Joker never seems out of place when paired against other foes, such as Superman, Wonder Woman, or Green Lantern. His chaotic persona makes it easy to believe that he would want to terrorize someone like the Flash.
https://screenrant.com/joker-black-flash-combo-villain-speedster-dark-multiverse/