I think it's a nod to the fact that Rorschach was based on Batman.
I'm fukking with you of course because you always get mad at people saying comic characters were based on Batman, especially when they aren't in the slightest.
Also, the pin is from the Comedian, and that means it's a nod to the fact he was based on the Joker.
The Comedian was based on the Charlton character Peacemaker. The smiley pin in the Bat-Cave is most likely because The Comedian has been masquerading as one of the 3 Jokers that has been running around the jacked up DCU.