Well, logically, the highlight is wrong, as you say. Such a hat would probably produce a huge cast shadow most hours of the day. That's the artistic freedom you must allow yourself; in return you get distinguishable facial features and some sort of sense of shape.
To have any sort of lighting and shading will always be a matter of compromising, since the character will undoubtedly find himself in various environments with varying light conditions.
To have any sort of lighting and shading will always be a matter of compromising, since the character will undoubtedly find himself in various environments with varying light conditions.