It was originally intended to be funny because obviously the person wearing it was so cool (hey, they're wearing a Dork shirt, they must be cool), so the shirt was ironic and they were in fact not a dork.
The modern ironic wearer can wear it because they are aware that the original wearers were in fact dorks after all, and so the shirt is funny because it was in fact accurate in labeling its wearer a Dork.
But then, is the modern ironic wearer himself a dork or not? He's aware that it's dorky to think he's not a dork and the irony is funny, so that makes him a dork, but if he's aware and doing it ironically, then he's not a dork? But then it's not really funny, so he's just wearing a stupid shirt that he thinks is funny and ironic and it's not, so really that's pretty dorky.
So I enjoy the riddle of the dork shirt.