NORMAN ROCKWELL'S PATRIOTIC TIMES by George Mendoza, introduction by Ronald Reagan (Viking: $19.95). Mendoza has combined various quotations and Rockwell paintings to produce a volume that resembles a scrapbook filled with old Saturday Evening Post covers and pages from a high school civics text. The quotes range from Walt Whitman's "Starting From Paumanok" to Tom Wolfe's "The Right Stuff," with a generous helping of patriotic songs and a sample of Mendoza's unintentionally hilarious doggerel. The illustrations display Rockwell's technical facility and love of the obvious, but it's difficult to understand why Mendoza classifies pictures of people hunting, playing baseball and shopping for Christmas presents as "patriotic." In his introduction, President Reagan concedes, "Our Nation has changed profoundly since the days of the America that Norman Rockwell so skillfully portrayed." And many of these images were already fondly imagined cliches when they were painted. The intervening years have not revitalized them. This heavy-handed paean to fictionalized Americana is enough to make the reader regret he had Yankee pot roast for lunch.