It's tempting to feel sad for Macabéa. She's hungry, sickly, ugly, and ignored. Totally sad, right? But she doesn't see it that way. So one of the questions The Hour of the Star asks is whether or not it's really okay to feel sad for someone else. See, there's something a little condescending about feeling sad for someone who doesn't feel sad for herself. And who's to say that there aren't people out there right now feeling sad for you? These are questions of perception, which, as it turns out, is something that Existentialism is super interested in. Can Macabéa actually make her own reality by being happy? Or is she actually sad without knowing it?
Macabéa is the only person in the book who isn't miserable.
Happiness and sadness are simply states of mind; they have nothing to do with what happens to you.