^No, I think that's just the running gag of his character -- that he's this totally amoral, id-driven guy that doesn't care enough about anything to be really affected by anything. He's not trying to better himself, so a spell that brings out his worst self doesn't change him. He's not trying to be anything he's not, so a spell that brings out his archetypal nature doesn't change him. And it's a source of frustration to the people around him who think he deserves to be taught a lesson but have to watch him perpetually breeze through everything without it touching him. Essentially, he's playing much the same kind of character that John Larroquette played on Night Court, the total scoundrel who's entirely content with being a scoundrel and never learns or changes.
Although it's possible that he's being set up for a really rude awakening in some upcoming episode when something finally gets to him.
Except that nobody tried to change Dan Fielding and no matter what happened to him he remained more or less the same, typical for a sitcom character. Ezekiel's running gag is getting alittle thin already, being a Librarian has changed anybody who becomes one, not to have any real changes in Ezekiel does seem to prove the characters are more like cyphers so far.