ONE LAPSE of taste diminishes the elegance and excellence of the new East Building of the National Gallery of Art.

The seven works of modern art, big pieces by big names, that have been commissioned to decorate the building - which needs no decoration, and, in fact, resists it - have about them something puffy, something almost, crass.

Together these new works cost $1.5 million. The Gallery, of course, calls them "monumental," but that word suggests quiet, nobility and grandeur, and many of these objects do not deserve such praise.

Grouped together in the light-bathed central space of the new museum, this collection of commissions seem less grand than grandiose. They battled with the building, with its scale and its mood, and usually they lose.

Joan Miro's huge tapestry might please at posterscale, but its whimsy is too slight and its colors are too garish for its enormous size. Seen in shifting daylight on a marble wall, it seems a heavy, mirthless joke. While Jean Arp's mushroom tapestry, blown up by the weavers from a 1950 silkscreen, seems a harmless "supergraphic," Tony Caro's sculpture, made of welded steel scrap, frets upon its balcony like some frazzled Juliet. The Caro might look tough in the white-walled anonymity of a conventional museum, but here it seems to yap at the imperturbable geometries of Pei's design. The Caro fails as decoration; exposed upon its ledge, it seems an over-nervous, reiterative mess.

Two of the works commissioned - "Knife Edge Mirror Two Piece," the big outside bronze by Henry Moore which guards the western door, and the even-bigger mobile by Alexander Calder which animates the space between the bridges and the skylight - are vastly more successful.

But even these two pieces, effective as they are (and the new Moore is a presence), carry an aura of cliche.

Too many public buildings, the Hirshhorn, for example, or New York's Lincoln Center, or the St. Louis airport, are already guarded by the works of Henry Moore. Paired reclining lions, which in old days used to flank broad stairs, thrones, or bridges, may have fallen out of fashion, but Henry Moore's large sculptures - which also call to mind the regal and the shaggy, elemental nature and imperial might - have now assumed their duties as the patient guardians of our museums' doors.

The Moore, an enlarged mirror image of a sculpture he designed in 1962 for a London park near the House of Lords, is a $500,000 gifts to the museum from Washington's Morris and Gwendolyn Cafritz Foundation. They also bought this city the black stabile by Calder that stands beside the Museum of History and Technology nearby on the Mall.

Our enjoyment of the Calder in the new East Building might, perhaps, be greater had all of us not seen so many other Calders so many times before.

His signature, like Moore's, is enormously familiar. Calders, too, are found in office buildings, airports, and innumerable museums. The Gallery's new mobile, 29 feet high and 70 feet wide, is based upon a little model that the artist made just before he died. Of all the indoor pieces commissioned for the Gallery, the Calder is the only one big enough to handle the scale of the hall. But it has paid a price for its enormous size. Its forms recall the artist's hand, his tinsnips and his pliers. Sweeping overhead, it seems, despite its playful grace, somewhat overblown. So, too, do too many of the other works commissioned.

With the exception of the Henry Moore, all of them are gifts of the Collectors Commitee, a group of 78 patrons and art buyers who so far have contributed more than $1 million for the Gallery's new art.

"Reconciliation Elegy," a 31-foot painting by Robert Motherwell, is the latest, and the largest, variation on a theme on which he has been working since 1949. A number of its predecessors, borrowed for the summer, will be on view together when the new East Building opens on June 1. Motherwell's best pictures are haunted with a dark, oppressive power. His newest work, in contrast, is anything but ominous. It appears to be a study in enervated nuance; beside its heavy-breathing brethren it seems pretty, almost chic.

The seventh work commissioned, a not-quite-minimal sculpture designed by James Rosati, also seems to be inadequately vigorous, overly refined. Installed somewhere else, the Rosati, like the Caro, might seize one's attention. But here it must compete with the crystalline geometries of Pei's new building, and it doesn't stand a chance.

Like most of its commissions, the Gallery's Rosati is a blow-up of a smaller work, a sculpture made of zinc which Rosati gave the Gallery in memory of his friend and champion, the late scholar William C. Seitz. It was Seitz, writing on Rosati, who once noted that "because of the oversize absurdities that fill so many of the world's parks and squares, the term 'monumental' has almost become a joke."

When the new East Building opens, its splendid central hall - with its windows on the Mall, its tetrahedral skylights, its sheer walls of gray marble, its balconies and bridges - will be acknowledged as one of this nation's grandest public rooms. But it is not, at least not yet, a room that's kind to art.

Of the commissioned works within it, most make the same mistake. They have left the realm of man to try to fight it out on the scale of the building. Only the Calder and the Moore pull it off.

There is one work in the hall, though, that gives hope for the future. It is statue by Maillol, an almost lifesize nude, who standing on her pedestal seems perfectly at home.

Pei has learned that a major Rodin sculpture show - it has not been announced - is already being planned for the museums central hall. Pei feels, I think correctly, that the master's statues need not fear that skylit space. For unlike the lump of stone, half carved by Noguchi, now lost there on the marble floor, the statues of Rodin are awesome works of art. And unlike the yelling Miro, or the borrowed pumpkin-orange sculpture by Alexander Liberman (which seemed harshly self-indulgent when it stood outside the Corcoran, and seems no sweeter now), the statues of Rodin have a spirit that's humane.

Though someday it may welcome first-rate works of sculpture, Pei's spacious glass-roofed hall was not built for paintings. It is, instead, an indoor park, a breathing space for visitors, an orientation hall analogous in function to the neoclassical rotunda of the Gallery's old building one block to the west.

"The rotunda," says J. Carter Brown, the Gallery's director, "is not a place for showing pictures. We tried it once, with 'Whistler's Mother,' which looked awful. The rotunda, and the garden courts, and Pei's skylit hall, serve another function. The grandeur of those spaces is, I know, sheer stage set, but that grandeur has a purpose. It lifts you by your coat collar out of your stock responses.

"Compared with the performing arts, the visual arts have a tremendous disadvantage. You are prepared for the theater. You've bought your ticket in advance, changed your clothes, eaten a good dinner; by the time the houselights dim and the stage begins to glow, your barriers have relaxed. Something very different happens in museums. You walk in off the street, usually in daylight, and there the pictures hang, one next to another. But you mustn't bump against works of art too casually; their truths are too distilled. Pei's space was designed to let you start your motor; his hall prepares the viewer to look at works of art."

The East Building is, of course, more than merely stage set. Dozens of galleries, of different moods and sizes, surround the central space. When the new museum opens, its galleries will house 1,200 works of art, some as large as walls, some smaller than a postcard. But before the viewer sees them, he must pass through the huge hall.

And that presents a problem. If we are meant to pause before we experience the Gallery's exhibits, then how are we to read the large commissioned pieces one sees from the streets?

Brown suggests these works are, at least in large part, architectural decorations that have been commissioned from modern art's "old masters" to complete the building. He says that, "They represent a totally exceptional approach to the way this institution collects works of art.

"We are not about to begin commissioning new pictures. It is against our policy to purchase works created in the current generation. Nor do these commissions reflect our exhibition policies; they are not our usual forays into aspects of art history. What Pei and I were gunning for was a new combination - a 20th-century gesumptkunstwerk - something that might occupy the land between pure art and pure architecture."

But Pei and Brown have not entirely succeeded. The Miro on the wall, the Caro on the ledge, and the other works commissioned seem more than merely ornaments. To the viewer off the street, these objects serve as signs; they seem to be a preview of the art that is to come.

As such they mislead, reinforcing the common misconception that the new East Building is a museum of moder art.Also they imply that they have met the tests of time passed by other purchased works in the Gallery's collections. That is not the case.

Perhaps even more importantly this collection of commissions suggests, one hopes mistakenly, that the Gallery will only seek big, and "safe," and costly works when buying modern art.

There is nothing at the Gallery more precious than its scholarly integrity. When Brown says that the Rosati "looks just right," that the Liberman is "ideal," and that "we all fell in love" with the big Miro, or when E.A. Carmean, his curator of 20th-century art, contends that the Caro and the Mothewell "are the best works in the bunch," one can only hope they are being diplomatic.

The Gallery has yet to make its reputation in the field of modern art. Its most recent modern show, of the cutouts of Matisse, was intellectually, historically and visually a superb exhibition that met the very highest curatorial standards. Though its new Moore is a triumph, and its Calder is a treat, the same cannot be said of the Rosati-Caro-Miro-Arp-Motherwell-Noguchi-Liberman exhibit that will open on June 1.