I think the message that people are getting from it, at least from the first half of the book, is "Don't let your misfortunes get you down." The back of the book has this quote: "'I may have been swallowed,' said the duck, 'but I have no intention of being eaten.'" But if that was the real message, then they absolutely would not want to stay in the stomach of the wolf at the end. The most accurate message I can get from the book is: "People in bad situations become so acclimated to them that it doesn't seem bad to them anymore." They actually start to like it, as in Stockholm Syndrome.
Message
The life of a parasite is awesome?