BEWARE THE DECORATIVE EXCESS that leads to violent revolution. At once aesthetic and political, this cautionary tale provides the standard explanation for the relationship between the exuberant usable arts of the Rococo and the stern history paintings of classicism, as well as between monarchy and modern democracy. It is now on persuasive display at New York’s Metropolitan Museum of Art, in two exhibitions: “Inspiring Walt Disney: the Animation of French Decorative Arts,” and “Jacques Louis David: Radical Draftsman.” As I entered the “Disney” exhibition, I overheard a caretaker ask a tiny girl