Norman Rockwell's America, Dulwich Picture Gallery, London

Lord, but zeitgeists are boring.

A year of economic downturn and it's all aboard the nostalgia express to Downton Abbey and 1912, a time before such worrying things as religious tolerance and the National Health Service. So where more natural to turn, in these dark days, than the United States of America, a country still largely unburdened by such things? That, I assume, is the logic behind the Dulwich Picture Gallery's sudden taste for 20th-century American art, the only other rational explanation being infiltration by the CIA.

Let me explain. The 20th-century American art I'm talking about is not that great strand which begins with Jackson Pollock and continues, much mutated, today, but the other American art, typified by Andrew Wyeth and his family. The Wyeths were given a show at the DPG in June, and now it is the turn of Norman Rockwell: the laureate of mid-century Middle America, of hands grooved by work, sailors, pot-bellied stoves and fluey men being dosed with linctus by women in aprons. Apple pie seems seditious next to Rockwell, white picket fences decadent. This is not to sneer. More than any other illustrator of his day, Rockwell showed Americans as Americans wanted to be shown, as plain, self-sufficient and white.

Which is to say that the director of the DPG is on the nail when he says in the catalogue, "Of [Rockwell's] greatness as an illustrator, there can be no doubt." Genius as an artist is not measured by audience size; genius as an illustrator is. Rockwell's following in Life, Reader's Digest and, pre-eminently, in the Saturday Evening Post was vast, the kind of numbers a chat-show host today might dream of. Where the DPG's director is on shakier ground is in admitting to an "epiphany" with Rockwell, citing in justification the critic of The New Yorker. "Rockwell is terrific," wrote Peter Schjeldahl a decade ago. "It's become too tedious to pretend he isn't."

Really? I wandered around the DPG's show feeling extremely tedious, and I wasn't pretending. Ex post facto, Rockwell has to be terrific: you don't get that kind of brand recognition as an illustrator without being so. Working backwards from numbers, Jack Vettriano is terrific, too. Would you want the director of a great public gallery owning up to epiphanies with him?

The question, really, is about art and its hierarchies, about showcasing low art in a high-art context and the motives for doing so. Let's be clear. Norman Rockwell was very good at what he did. His images have a charm that is all the more charming for being lightly self-mocking. Like N C Wyeth, he loved Dutch genre painting. Unlike the dreadful Wyeth, Rockwell sees the ridiculousness of drawing 1940s America as though it were 1640s Haarlem. His Santa's Workshop is funny in part because of Father Christmas's grin, but partly because the man who drew it knew he was conjuring up la-la land. Faced with the unpleasantness of a coalition government and politically motivated spending cuts, we turn to Norman Rockwell. But Rockwell himself, faced with a Depression and world war, was already looking backwards, and laughing.

That is his trick. Underpinning all his illustrations is the admission of impossibility. To be possible is to be real, and who needs that? The family in Rockwell's famous Freedom from Want poster is like William Mulready's Seven Ages of Man, except that every age is physically perfect, from tow-headed infancy to necktied senescence via Colgate-white adulthood. That the prosperity (far less the teeth) that Rockwell depicts will have applied to a small minority of Americans in 1943 is neither here nor there. To criticise him for hymning the white middle class is like attacking Canaletto for painting Venice. Rockwell's business was not to be a realist but a propagandist, in a sense a pietist. And his vision works.

Bitter old Brit that I am, I want to believe in his Santa, to fish from his dinghy, to be spoon-fed cough syrup by a calicoed old biddy with a steel-grey bun. What I do not want is to have this done to me at the Dulwich Picture Gallery. The DPG was built by Soane, owns Raphaels and Poussins and Rubenses and Rembrandts. It also has a pretty good collection of Dutch genre paintings, and some vague case for this show might have been made by juxtaposing these with Rockwell's illustrations. As it is, I see no reason at all for it being here, unless it is to suck up to the gallery's American Friends or to please grey-suited men in Langley, Virginia. No more American illustrators, please. In the words of Mrs Reagan, just say no.

(020-8693 5254) to 27 March

Next Week:

Charles Darwent leafs through books illustrated by Pissarro fils at the Ashmolean in Oxford