Popular culture is absolutely a legitimate field of study in academia. There are at least three reasons why.
First, if anthropologists can study other cultures, why would it not be valid to study our own culture? Our own popular culture is no less interesting than the ways of a foreign culture.
Second, understanding popular culture is important to understanding the strengths and weaknesses of our country. For example, sociologists might need to understand our popular culture in order to understand whether it is contributing to a "decline" in our society. If we are to understand problems that occur in our society (like the current problem of a decline in marriage and an increase in out-of-wedlock births) we need to understand what our popular culture is like.
Finally, understanding popular culture today is a step towards writing the history of our time. History books tend to have sections about the popular culture of any given era. One day, history books will have such sections about our own time. Therefore, it is valid to study that today.