When applied to fiction, the term Lowest Common Denominator means appealing to as many people at once as possible.
There are varying interpretations on what this implies. Taking an upbeat viewpoint, it might be admirable; making stories that most people can relate to, stories that all human beings can understand in relation to their own lives. If we think of media as communication between the creator and the audience, the most successful communication would be the one that is understood by the most people.
From a more elitist, downbeat point of view, if we assume that a significant percentage of Viewers Are Morons, it can mean a work that is made to appeal only to the Great Unwashed. Instead of more "worthy" audiences, it is marketed to Joe Sixpack and all his family out there in Flyover Country, taking out everything that anyone more educated might appreciate, reducing it to its most shallow parts.