During an interview with GamesRader+, director of Joker Todd Philips mentioned that Joaquin Phoenix suggested a bloopers reel for the end of the movie. Phoenix thought it would be funny, and of course offer much juxtaposition to the film, which was dark and gritty.
If you watched the movie, you would know that this never came to be, which was probably for the best, considering how tonally different it would come across. This reasoning extended into the lack of a post credits scene as well. Even though it has become standard fare for comic book movies to include a post credits scene (or scenes), ultimately, Philips felt that including one “would seem wrong, and a little too light for [him]”. I wish I had known this before I waited around after the movie.