should you joke about your mental illness to others?

I work as a barista & today this regular came in & while I was making her drink she mentioned how she had to drive into the cities today. I asked her what for & she told me that it was for a doctors appointment. She proceeded to mention how her therapist thinks she's crazy for going out that far when we have a good doctor in town. I was responding normally & trying to keep a good conversation & then she went into saying "I have a mental illness can you tell? Probably not my psychotic hasn't come out yet." Then she laughed about it like a joke. It made me so uncomfortable & I didn't know what to say. I hardly know this lady, she comes in, I make her drinks & that's it. In a way it kind of offended me too because I struggle with a few mental illnesses myself & I don't think it's a joke at all. It's something that can seriously effect someone's life & can very very challenging to deal with.