I would suppose that the moon would indicate someone looks a bit out of the ordinary. What some people would say as "crazy"--which could mean they just look different from what is considered 'the norm'.
Perhaps though it could be as simple as a person who has a nice round face, big eyes and smile even. Thinking upon this when I read your request made me think of quite a few people I know whom had a look similar to this. They all, too, come to think of it, always had a big wide smile too on their face, sort of like a picture I know I've seen somewhere of a smiling moon. I'm going to search for it online and hopefully find it. At any rate, these are a few ideas anyway.
such as these, perhaps?