"Fine, thank you. How are you?" or "I'm fine, how are you?" That (more or less) is the normal, polite, expected response. I don't see how it can be a joke, except that it is such an automatic "stock" answer that it means nothing. Typically the next sentence would be some comment on the weather. One could joke about that, too, because it's so stereotyped. The polite answer to "how are you?" is almost always "I'm fine." A friend of mine says he was "taught" an etiquette rule: "Don't tell your friends about your indigestion: 'How are you?' is a greeting, not a question."

That is acceptable. I do not see a problem with that response.