Decoration is Sin - or so said the Mid-Century Moderns, holders of "advanced taste."

In the 1940s, a certain woman of exquisite, if avant-garde preferences, received as a wedding present an entire set of highly ornamented Tiffany flatware. She asked a jeweler to give her a price for removing the decoration. All that saved the Tiffany was the price of the desecration.

The ideal was Phillip Johnson's house in New Cannan, Conn., undoubtedly the most exquisite minimal design ever made all glass walls, five chairs, one leather bench, one bed, one desk (glass) and one picture mounted on an easel.

During the heyday of the Modern school, neat - not gaudy - was the rule. Those of good taste advised people to keep all their ornaments in boxes in the basement and to bring out only one exquisite piece at a time, for concentrated contemplation in the Japanese. Conspicuous consumption was declasse.

All this was a reaction to the excesses of the Victorian era, when ornamentation of every period was piled helter-skelter atop poor, defenseless art objects, without regard to suitability or even ability to sustain it.

In 1977, the pendulum swings again. Next week, for instance, both Sotheby Parke Bernet and Christie's, the two big New York auction houses, have scheduled large sales of Tiffany lamps and other decadent decoration in the art nouveau taste. In Washington, a show of Faberge object d'art at the National Geographic Society attracted 1,600 people on the first day. People are reexamining ornament, rediscovering its voluptuous, sensual, delicious pleasures. Style, like lfe, is either feast or famine.

Probably the richest feast of collections in the country is that of the Cooper-Hewitt, the Smithsonia's National Museum of Design in New York City. During our "less-is-more period," the Cooper-Hewitt's treasure rooms have kept the faith, and the ornaments, secure against the day when we would once again appreciate the lovely, the lush, the luxurious. The collection has been packed up, loaned out or under restoration from the time the Cooper-Hewitt Union gave up the collection in 1963. The museum was reorganized under the auspices of the Smithsonian and the directorship of Lisa Taylor.

After a year, and 250,000 visitors, in its new home in the Carnegie Mansion on Fifth Avenue, the Cooper-Hewitt Museum is showing 400 objects from its collections - the first show from its own holdings since its reopening. It's about time. The earlier shows this year have dealt with other subjects, other collections, and while the Brighton Pavilion show was a marvel, most design buffs were becoming anxious to see the Cooper-Hewitt's own marvels.

"More Than Meets the Eye" opens Tuesday and continues through January. Taylor likes to think it provides an index to future shows. In an effort to give an air, however spurious, of seriousness to this love feast, the objects have been organized into categories: surface embellishment, sources of design, fun and fancy, construction, patrons and clients.

(Most people won't pay any attention to such artificial classes. Too often the problem with design people is that they work so hard at word and visual tricks, to convey their high-minded purpose and deep philosophy, that they stand in the way of what we really want to do: look at the object. Just give us a good strong light, a Plexiglas case to keep us from stealing it and stand back, please.)

The most beautiful objects to the current show may well be the 1905 desk of Oriental woods, inlaid with mother-of-pearl and brass by Carlo Zen, made (one is tempted to say "grown" because of its organic decoration) in the Stile Liberty manner (the Italian version of art nouveau). Christian Rolfing, a long-time Cooper Hewitt curator, says that not too long ago someone walked in with a bundle of sticks and said, "This once was a Zen chair, do you want it?" It was the chair that went with the desk. And yes, the Cooper-Hewitt did want it. Now beautifully restored, the chair is living happily with its desk. Another Zen design is a side chair made of fruitwood with incredibly delicate inset flowers of mother-of-pearl, brass and pewter.

The French art nouveau master, Hector Guimard, is represented by a side chair from the dining room of his own house, offering art nouveau buffs the chance to compare the two masters. If you thought Guimard was good, you'll think Zen is the zenith.

American art nouveau is represented by an unusual piece in a far simpler style than you associate with the name Louis Comfort Tiffany: a blown glass-footed bowl in which four flowers and their stems are forever trapped in what looks like a crystal pool.

The Cooper-Hewitt has a great advantage over everybody else: It often owns various stages in an object's life, not to mention its friends and relatives. And although it no longer has the wealthy sisters, Sarah and Eleanor Hewitt, as patrons, it continues to scrape up the cash to acquire important works. In the present show are 12 rare pen-and-ink and watercolor designs for Sevres 18th-century porcelain. Several objects made from these designs, including a tureen and a salad bowl, recently were bought from two Paris dealers. The 1926 Lobmeyr vase from Vienna is there in sketch and in actuality.

The epitome of art moderne are the sketches for a dining room in an apartment at 950 Fifth Ave. by Jules Bouy, a Belgian designer (1872-1937). The elegant doors and the marvelous lacquered screen from Mrs. Solomon Gugenheim's apartment, designed by Sepherin Soudbinine in what has been called the "Aztec Airways Style," are also in the exhibit. Though the Bauhaus was preaching sleek simplicity in the '30s, there were those who held their ears. A rectangular 1935 French table of shagreen (leather textured by pressing small seeds into untanned hides) with bone drawer pulls has to be the most devious way to cover an otherwise plainly constructed table.

Kinship of design - the Seven Seas motif - can be seen in the 1802 sketch, a Beaux Arts "project for a public monument," by Jean Pierre Louis Laurent Houel; a wallpaper window shade from the New York Crystal Palace in 1853; and a much later three-dimensional globe in metal.

The Cooper Hewitt, at the risk of being considered either flighty or a rare bird, has what well may be the world's largest collection of birdcages. (Perhaps that's a reason Smithsonian Secretary S. Dillon Ripley, an eminent ornithologist, wanted the museum.) Two of the best are in the show: a triple pleasure dome made of crystal and - captured inside - a glass bird; and a 1765 gothic revival English birdcage of mahogany, wire and sheet metal. Every bird should have one of each.

That's only a small section of the show. Among other marvels (put together by Rohlfing, Elaine Evans Dee, Dorothy Twining Globus, Gilian and Milton Sonday) are an early (1940s) plywood chair by Charles Eames for a child friend; a Belter laminated chair, a chair from the Imperial Hotel in Tokyo by Frank Lloyd Wright; a model for a Marmon Sixteen Sedan by Walter Dorwin Teague; an 18th-century bidet; Louis Desprez sketches for a a tomb; and more, more, more . . .