The problem with saying that one comic book character is a rip-off of another is that the vast majority of comic book characters have evolved quite dramatically from their creation, and two characters that are similar today does not mean that one was copied by the other.
With the Joker and Green Goblin in particular, while the joker started out as a very dark character he evolved to become a very campy character about the time the Green Goblin was created (mid 1960s). This was in part a response to the senate hearing on juvenile delinquency in the 1950s which blamed comic books for all problems of young people.
And to think they still try to do stuff like that even these days.
"I don't understand how someone could like Tolstoy and Dostoyevsky, but not like Twilight!!!"
"Last book I read was Brokeback Mountain, I just don't have the patience for them unless it's softcore porn."
(The Voice of a Generation and Seece)
"If you cant stand the sound of your own voice than dont become a singer !!!!!"