Zim is by Jhonen Vasquez (dont really care about the spelling) and was on Nickelodeon
Billy and Mandy is by Maxwell Atoms and is on Cartoon Network.
completely unrelated projects...
the only possible link is the song Brains in Billy and Mandy was done by a guy named Voltaire, who draws angsty goth comics, as does Jhonen... the difference is voltaire tends to make fun of goths rather than make them out to be the coolest thing in the universe...