Scrawl "Merry Christmas" across the bottom of any painting by Bob Timberlake and you've got it: the perfect Christmas card, vintage 1945.

But hang these snowy barnyard scenes and autumn barnyard scenes and summer barnyard scenes at the Corcoran, and you've got the most embarrassing show the museum has come up with in years.

Not that there's anything wrong with barns, or with Timberlake, who, 16 years ago, saw a story about Andrew Wyeth in Life magazine and was inspired to begin painting in his spare time. Now 44, he enjoys a large reputation as a watercolorist in his home state of North Carolina, and throughout the southeast. Since he gave up business for painting full time 11 years ago -- on the advice of Andrew Wyeth himself -- Timberlake has had several shows in New York, where his works sell for more than $20,000 apiece at the Hammer Galleries. That's Hammer as in Armand Hammer, the Corcoran's benefactor.

Timberlake is also sufficiently well-connected to have gained himself an audienced with President Jimmy Carter (after being appointed "Official Artist of Keep America Beautiful") and last week met with President Reagan. His art has been featured in Reader's Digest, and there is a book out called "The Bob Timberlake Collection," with an introduction by Charles Kuralt.

There are many artists like Timberlake, who paint similar scenes of the vanishing American farm and its homely charms: weathered wood, baskets of pumpkins, barrels of corn husks, ducks and decoys. Unlike Wyeth, however, what they paint has no depth beyond a contrived nostalgia -- the kind that sells holiday greeting cards. What they produce is not the richness of vanishing rural American, but the worst cliches in contemporary American art.

The question is: Why pick one such artist and give him a show at the Corcoran, whose mandate is: "Promotion, as well as encouragement of the American genius"?It isn't hard to guess, and the implications are disturbing. Should a museum's responsibility to maintain standards ever be allowed to lapse in the face of a need for money? If it weren't so hot in the Corcoran -- which still has insufficient funds to complete its airconditioning program -- the answer would be simpler.

It must be said, however, that certain aspects of Timberlake's paintings do have their charms. He clearly excels in rendering the softness of green grass, sunlight pouring through translucent leaves, yellow wildflowers on a mossy forest floor. There are other seductive passages: a spotted blue enamel pot that seems so real it nearly jumps out at you; an exquisite little bowl of fresh raspberries in various stages of ripeness.

But even here, Timberlake drags in the weathered wood as backdrop for his berries, ruining their innocence with his favorite props. There is actually on view in this show a painting of a Raggedy Ann doll with a Christmas wreath, the sort of amateur fare that turns up at December church bazaars. Conceptually, Timberlake seems compelled to reduce everything to sentimental mush. His friends -- or at least the Corcoran curators -- might have saved him from himself by at least cutting out such works and reducing the entire show by two-thirds.

There are three wholly fresh, original paintings in this show, and they suggest what Timberlake might do if he could screw up his courage and liberate himself from the slavish pursuit of the Wyeth look. Two are wholly charming renderings of ducks, each one surrounded by splatterings of paint, suggesting at least a yearning toward originality.

The third painting is a sundrenched watercolor showing a tropical beach with palm trees and beach grass bending in the wind. There's still the requisite wooden hulk, but this time in refreshing form: a wrecked yacht. The change of venue, miles away from Wyethland and Wyeth subject matter, makes clear that Timberlake really has a way with light, and could paint far more interesting works if he could climb out from behind all that weathered wood.

The show will continue through August.