I agree, I can barely stand listening to Americans trying to do Australian accents, because they're always so off. The one time I heard an accurate Australian accent in an American cartoon, I was so happy. The character was voiced by an Australian.
But I don't mind if the accent is part of the joke, which is how I see it with Jurgen. I have a theory that Jurgen is actually American or something and pretends to be German because he thinks it makes him cooler, though that doesn't explain the zombies.