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The perpetual indeterminacy of his seriousness is not a bug, it's a feature. The unpredictability keeps everyone and everything off balance, which makes him uniquely powerful.
Everything is a joke, the threat of suffering, the fear of suffering, or even the reality of suffering.
People support him in part because they want to be insiders, safe from being the target of his jokes.
But there is no insider with him.
The Joker has been made the mayor of Gotham.
No-one is safe.