I think the word shallow could be looked at metaphorically in the sense that it indicates a person who is not connected with his deeper true self and therefore clings to whatever he thinks is the best way of acting.
If you don´t recognize yourself as an intrinsic part of life and see your connectedness to other people and life in general you will base your perception of self-identity from your education/friends/work/social status and so on. Then you will be always seeking fulfillment outside of yourself and become easily "brainwashed by the media" or your social roles. The more you are identified with your roles and not your true self the more shallow you become.
I think that in a sense we are all shallow in some way but we recognize it more in others than in ourselves. The cure and the best way to cultivate depth of personality and character is then to listen to your inner self, your heart, your soul, your buddha-nature (whatever you choose to call it) because then your action will be based on wisdom and compassion, spreading love, joy and peace to the world around you.