One thing about Yeardley Smith: No revue should be without her.

Looking rather like a mad mushroom with a button nose, she is the consistent delight of "The New Improved Bride of Sirocco," the somewhat inconsistent collection of songs and sketches by Tim Grundmann that opened Sunday night at the New Playwrights' Theatre.

Like the four other performers cavorting on the circular platforms that have been arranged to form an Art Deco staircase, Smith occasionally finds herself trapped in some less than inspired material. While her companions simply put up a brave front and try to muddle through, Smith draws on two secret resources, sugar and malice. They may not save this show from being more than fitfully entertaining, but they do make it hard to take your eyes off her.

Her face, at first glance, is dominated by what appear to be two self-rising dinner rolls that turn out to be her cheeks. Her eyes are no more than dots, really, and her eyebrows could be misplaced commas. It's one of those faces that might have been designed by a cartoonist --which may be why she's the perfect choice to spoof Annie or Rhoda (the Bad Seed) or even Cheetah (Tarzan's chimp), all of whom figure in the proceedings.

By popping her eyes wide open--and making them, I swear, into miniature sunflowers, Smith is the essence of sunshine, as it's generally depicted on a kid's birthday card. But let someone cross her pint-sized will and her mouth gathers into a pout, her eyes all but vanish in a squint of displeasure, and steam shoots out her ears. There's a touch of Shirley Temple here, a suggestion of Dennis the Menace, along with a whisper of Rasputin.

In short, she's an original presence in a show that is longer on promising ideas than it is on hard accomplishments. "The New Improved Bride of Sirocco" is a virtual grab bag of Grundmanniana--some of the numbers are indeed new, some have been reworked from "Bride of Sirocco," Grundmann's revue for 1976. As in most grab bags, the gewgaws are of wildly divergent worth. "Il Tarzani," the daffy centerpiece of the original "Bride," is back. (In it, Grundmann retells the saga of Tarzan and Jane as a Puccini opera.) So are "Papal Pageant," an irreverently funny sketch in which the selection of the pope is equated with the Miss America pageant, and "Lust for Locution," the life story of Peter Mark Roget, the thesaurus-maker, as it might be dramatized on "Masterpiece Theater."

New are Grundmann's views of heaven, hell and purgatory for actors --hell being a place where "every song is written out of your range," heaven being where "you write your own review" and purgatory being not unlike the New Playwrights' Theatre itself. Anticipating the fall season on Broadway, he's come up with "Rats," three portraits of society bores, the music and lyrics of which are as sophisticated as anything he has yet done. Still, unevenness prevails.

The problem, it seems to me, is that Grundmann is at a crossroads. His early revues were heavily improvisational in spirit and much of their fun was their antic tone, born of a moment's insanity. As a craftsman, however, he is no longer willing to settle for mere flakiness. His lyrics are increasingly more considered, and his melodies more intricate. And yet, the old loose-leaf nuttiness still serves as the point of departure. One gets the impression in "The New Improved Bride" that Grundmann is trying to go forward and backward at the same time: break new ground and recapture prior innocence.

Grundmann's first revues were staged on a shoestring, which definitely enhanced their spontaneity. But as production values improve at NPT--and this time they're nightclub-spiffy--they put new demands on the material. What was highly inventive when done on a dime doesn't look quite so inventive in fuller surroundings. Sometimes, a stool and a spotlight are the best set. With the mischievous Yeardley Smith on hand, one is less inclined to lament the absence of Dana Vance, who galvanized the original "Bride of Sirocco." And Steven LeBlanc, a chunky comic who can make smarminess cosmic in its dimensions, is a sure asset, as Tarzan, the pope-to-be and a talk-show rat. The others--Chuck Tobin, Carol Tate and Diana Ridge--may be pleasant enough, but their talents are hardly transcendent, clearly riding or falling on the merits of the individual numbers.

With a half-dozen or so shows, Grundmann has earned himself a reputation for being one of NPT's most distinctive talents--a Cole Porter, you might say, for the acid generation. But maybe it's time to move on. He's entirely too talented to settle at this point in his career for the sweet pleasures of harking back. Even if it's only for a hot summer's night. THE NEW IMPROVED BRIDE OF SIROCCO. By Tim Grundmann. Directed by Harry M. Bagdasian. Sets, Lewis Folden; lighting, Richard Moore; costumes, Mary Clare Gromet; choreographer, Anne Reynolds Day; with Steven LeBlanc, Chuck Tobin, Diana Ridge, Yeardley Smith, Carol Tate. At the New Playwrights' Theatre through Aug. 15.