I think that exams like IELTS require you to speak in a more formal way... But I guess that if she made such a joke it wouldn't be a problem as long as she explains later on, in "formal English", what or why she meant it... Actually I think making a joke (nothing disrespectful though) can even demonstrate the ability you have with the language - again: as long as it makes sense or if one says "I'm joking" after the joke and then says something else more seriously.