Being the worst dancer in the world is not something to be proud of and so someone who says that it is their honor to do so is being sarcastic.
Consider the staetement "I had to dance in front of everyone at the wedding and I can't dance well.". Based on this statement, is "It's my honor to be the worst dancer in the world and put up a magnificient act by dancing in front of everyone at the wedding." a true statement? Yes/no/unknown