The Hellboy comic book has that sort of humour mixed in with dark storylines, and so it transferred into the movie. It'll be in the second one too. There is a vast difference in the tone of Del Toro's material, I think part of the reason he was chosen to direct is because he has a strong visual style, as Hellboy does, and he has a great capacity for deep and penetrating film making/story telling. He also has a sense of humour, and so the Hellboy movie really couldn't have been directed by anyone else.