Calling someone shallow is actually saying the person lacks depth, tending to look at situations superficially. To judge a person based solely on physical appearance is to show little depth of character, or to appear shallow.
Shallow people are described as being vain and focused on physical perception. Their perception of life is based mostly on beauty, property or money. The movie "Shallow Hal" starring Jack Black portrays an example of a shallow man who soon is forced to realize that there is more to the women he encounters than how they may first outwardly appear to him. His perception of them is reversed so he can focus on what is inside rather than their physical characteristics.