He is not. The movie’s director, James Gunn, has previously said that the Joker “wouldn’t be helpful in that type of war situation [the movie’s setting]”. It seems that while fans of the clown prince of crime might be disappointed, this could be a blessing in disguise; rather than relying on familiar faces to sell the premise, Gunn has assembled a cast of characters that mesh well with one another and that can stand on their own merits.