What was the point that the author was trying to make in the book The Prince and The Pauper?

It would not be credible if a king and an adult pauper were to exchange clothes and identities in a story, but the principal characters in The Prince and the Pauper are just a couple of kids and do it as a prank on a whim. The Prince soon discovers that he is treated with extreme cruelty and hostility, while the Pauper discovers that he is treated with the greatest respect because everybody thinks he is the Prince and in line to be the next king. The moral of the novel is that most people are hypocritical and will treat you according to what they think you might be able to do for them.
Mark Twain became cynical about humanity as he aged. He shows this cynicism in works like "The Million Pound Bank Note" and "The Man That Corrupted Hadleyburg," as well as in his great novel Huckleberry Finn. Mark Twain was once quoted as saying:

All that I care to know is that a man is a human being. That is enough for me; he can’t be any worse.