I had to pare down the question because I think that both Spiegelman's work and McLuhan's work represents postmodern thought in separate ways. At the same time, this is not going to be easy because McLuhan's ideas are both complex and intricate. McLuhan's phrase of "The medium is the message" means that how art or media is conveyed is as important, if not more important, than the content itself. Message and messenger are not opposites, but rather one in the same:
McLuhan proposes that a medium itself, not the content it carries, should be the focus of study. He said that a medium affects the society in which it plays a role not only by the content delivered over the medium, but also by the characteristics of the medium itself.
This is postmodern for a variety of reasons. On one hand, McLuhan's idea about medium and message represent the essence of deconstruction because it seeks to go beyond the traditional binary opposition of "medium" and "message." In McLuhan's idea, medium is the message. How a society understands the message is almost as telling as the message, itself. For example, Spiegelman's work, Maus, says more that it is a comic strip about the Holocaust than its content about the time period. The fact that more children have read Maus than Night by Elie Wiesel might speak volumes. McLuhan's idea is also postmodern because it forces the lens of analysis on personal identity and social reality, examining how both are dependent and independent of one another. This is postmodern because it seeks to locate understanding in both realms, and not one or the other. The fact that McLuhan's study is in media is also postmodern in that the movement was concerned with different ways to connect human experience to literature and to analyze different modes of being in the world.