Yes, the title is a play on Marshall McLuhan’s notion of a “cool medium.” Medium Cool is a souvenir of McLuhan’s association with the radical mood of the 1960s, depicted here by events surrounding the riots at the 1968 Democratic national convention in Chicago. The movie was controversial when it was released in 1969 — it received an X rating, which, according to the lore, was not for violence or sex, but for political content. People loved and hated it, sometimes simultaneously. An excerpt from Vincent Canby’s New York Times review:
The shock of “Medium Cool” comes not from the fiction, but from the facts provided Wexler by Mayor Daley, the Illinois National Guard and the Chicago police. In his use of these events and others, however, Wexler does seem to be somewhat presumptuous, attempting to surpass the devastating live show that television—Marshall McLuhan’s “cool medium” — presented as the Chicago riots actually were taking place.