The Joker is one of the only comic book villains to reach icon status, usually it's only the most famous of comic book superheroes that achieve that...like Superman, Batman.
This is partly due to the fact that the Joker was built upon an existing icon (the Joker card in a deck of playing cards), however, the character has taken that symbol and made it all his own not only bringing to mind the clever court jester, but also the clown as his modern replacement to create a strangely fun and likeable villain... so that most people cannot help but think of this
when they think of this