The most familiar form of art as cultural commentary is social satire, such as is found in the engravings of Goya.  The caricatural nature of the people, things, and situations in Goya's engravings is less a means of creating the impression that it represents social reality of the time, than a means of expressing a commentary on social reality.  "Que Sacrificio" comments on the sad situation of a young woman, represented as innocent, who is being sacrificed in an arranged mariage to an older, wealthy man, represented as a lecher. 

Manet's "Olympia" is parody that makes fun of a traditional form of art, the neo-classicial odalisque (see "Mme Récamier," David), and its glorification of the ideal woman by replacing the ideal woman with a prostitute and her exotic servant.   It uses a representation of cultural reality, in which the services of women were bought and sold, to parody the ideals with which people misrepresented women.

A more modern notion of art as cultural commentary is the theory that all cultures are constituted by discourses (ways of representing reality).  These cultural discourses construct what we, the painter, the novelist, constitute as real.  Cultural discourses thus construct society's and the individual's, consciousness of what is real, although we are not normally conscious of this because we imagine that we see reality as it is.

Dultural discourses that constitute what we call reality tend to reduce reality to stereotypes based on polar opposites:  master/servant, colonizer/colonized, white/black, male/female, Western/non-western, etc.

More recently, some critics see visual art as the product of the discourse that now dominates the world, especially since the fall of the USSR:  the discourse of capitalism.  The visual media is a major means by which this discourse constitutes what we and art represent as reality.   Roy Lichtenstein's, "In the Car," foregrounds this discourse by representing a cartoon but idealized representation of a car and riders.   We are the consumers of media images.   And because the media makes us see what the market wants us to see (or so the theory goes), it makes us desire what the market wants to sell (such as a fancy car) in order to gain the status which that commodity confers (being the people in the fancy car).   The market thus consumes our resources and, in a world where our lives are determined by our buying power, our lives.