This is the first reference in the book to what is more commonly known as “Socialist realism” (see Bookmark, page 91), the term for a narrow range of aesthetic values under Communist regimes, and the only officially approved type of art in the Soviet Union and many of its satellite nations for roughly 60 years during the twentieth century.
In Socialist realism, art is supposed to serve only the goals of socialism and communism, specifically the values and activities of the common worker. Other modern art movements such as impressionism, cubism, and aesthetics that were not strictly realistic, and that employed dream imagery, irony, and other distancing devices, were denounced as “decadent,” and more or less strictly controlled by state censorship. As Kundera’s narrator says, “art that was not realistic was said to sap the foundation of socialism….”
Although Kundera doesn’t explicitly say so, he would probably agree that Socialist realism is a form of “heaviness” against which the “lightness” of Sabina’s aesthetic and moral values runs afoul, just as Tomas’s political dissent will get him into trouble with the Communist government bureaucracy.