I'll just note that the traditional role of the 'Fool' character is seen as one that is able to challenge authority using the pretence of a joke; to that end, it becomes a symbol of anarchy and revolution, which is what Nolan's Joker was creating.
Literary examples would include the 'Fool' characters in Shakespeare's "Twelfth Night", "King Lear" and "A Midsummer Night's Dream" (Puck), E. A. Poe's terrifying "Hop-Frog", and the real-life example of the Jester at the court of a French King Phillip who was the only courtier capable of breaking the news of France's defeat at sea without being summarily executed due to the fact that he could joke about it.