Key themes in The Twits by Roald Dahl.
Dahl explicitly states that horrible thoughts make a face horrible — a person's inner ugliness shows on the outside. This is one of Dahl's most frequently expressed beliefs.
Both Twits spend their time trying to outdo each other in nastiness. The Muggle-Wumps' revenge is satisfying because the Twits fully deserve it.
The Muggle-Wumps are caged and forced to perform. Their desire for freedom is entirely sympathetic. The story ends with them free and the Twits imprisoned by their own foolishness.