When it comes to antonyms for the phrase "am serious about," one could consider phrases such as "am joking about," "am frivolous about," or "am insincere about." These phrases denote a lack of sincerity or seriousness, and can convey a sense of playfulness or lightheartedness. Alternatively, one could use the phrase "am indifferent about," which suggests a lack of concern or interest in a particular matter. Whatever the chosen antonym, it is important to consider the context in which the phrase is being used, as well as the tone and intention of the speaker.