I think it would be more common to say, "are you joking?" rather than "are you mocking?", although you might hear someone say, "are you mocking me [her, them, etc]." That said, joking and mocking aren't interchangeable with being sarcastic. Sarcasm is a specific way of joking or mocking someone or something in which the literal meaning of a person's statement is different than the intended meaning, usually opposite. For example, "Let's ask Jim. He is always right" actually means that Jim is never right if said with sarcasm.

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