Over the past decade, there’s one truth that liberals have been loath to admit: Donald Trump is funny. This aspect of his appeal prompts far less commentary than his far-right positions, his venality or his mogul’s bravado. But when you watch him at a rally, you can see he’s playing for laughs: jabbing at his opponents, doing crowd work, even being self-deprecating, sort of. Cicero could write a treatise on Mr. Trump’s use of irony, as he’s proved himself a master of humorous misdirection. Liberals tend to think that irony is a type of wit that is aligned with progressivism....