Joker does not have a post-credits scene, and director and co-writer, Todd Phillips, spoke to GamesRadar and explained why it was excluded from the film.
“The idea of a post-credits scene in this movie would seem wrong, and a little too light for me,” Phillips said.
“That wouldn’t have been something we did. But Joaquin
said it would be funny to put bloopers alongside the names like they did in the old days,” he said.
Joker isn’t the only comic book movie to buck the trend of a scene during the credits.
Source: IGN.com Why Joker Doesn't Have a Post-Credits Scene