In "Thieves," now at area theaters, Marlo Thomas betrays a haggard look that comes as something of a shock, at least to those of us who haven't been following her fortunes with close attention. Nevertheless, it might have helped to define the disillusion felt by the character she's supposed to be portraying, an incorrigibly girlish idealist and do-gooder who can't accept some of the obligations and disenchantments of maturity. It might have, that is, if Herb Gardner's writing weren't so incorribigly pretentious.

Thomas still flashes plenty of the big, sunny smiles that have remained her most distinctive feature - almost her trademark - since the days of "That Girl!" on television. The stills released for the movie protect this image by emphasizing sunny smiles too. It hasn't been possible to conceal unsunny moments from the movie camera, and when the Thomas beam fades or collapses, the effect is distressing. We seem to be watching her lose her stock-in-trade in the service of a vehicle offering nothing more substantial or attractive in return.

"Thieves" evidently began as a movie script, which Gardner then transformed into a play at the suggestion of Paramount, which helped finance the Broadway production before making this movie version. Marlo Thomas, Professor Irwin Corey (the movie's only positive attraction/ and Ann Wedgeworth have recreated their stage roles, while Charles Grodin, who directed the Broadway show, has been cast in the costarring role of Thomas' estranged husband.

Gardner recapitulates the urban blues already sung into the ground in Jules Feiffer's "Little Murders" and Neil Simon's "The Prisoner of Second Avenue," among other things. Thomas and Grodin play prisoners of York Avenue: Sally and Martin Cramer, who grew up together on the Lower East Side, are now drifting apart in their affluent high-rise apartment. She feels the young man she loved and married has grown too materialistic, while he feels the young woman he loved and married is refusing to face the dog-eat-dog realities of making a living and getting ahead.

It seems a wonder that anyone considered Gardner's material intriguing.His dialogue is unspeakably inflated, full of gaseous, cliched lamentations about the alienation and despair of modern urbanites and equally gaseous affirmations of the You-Can't-Beat-Us-As-Long-As-We-Love Each Other Crazy-Old-New-York We-Still-Love You Too School. The echoes become slightly bewildering: It's as if a radio were picking up scrambled transmissions not only from Feiffer and Simon but also from Clifford Odets and the film version of "You Can't Take It With You" and Gardner's own smugly affirmative "A Thousand Clowns."

Gardner may be guarding his own naivete by patronizing the characters, especially dear unworldly Sally, who's meant to be a touchstone of pure feeling. The contrast between Sally and Martin is illustrated by the fact that she still teaches in a tough ghetto high school in their old neighborhood while he has become headmaster of a private school. Even when she behaves dopily - for example, sending their furniture to the wrong apartment - we're supposed to find it adorable, since her follies are so goodhearted.

Although Gardner reaches for fancy, even heartrending, rhetorical effects, he lacks the lyric or social version necessary to create something touching or pertinent out of his perception of lonely, disillusioned Nice People. Gardner merely sketches while wearing his heart on his sleeve. When he plants Martin inside an abandoned movie theater to lament the lost heroes and pleasures of his boyhood, the effect is devastating in the wrong way: This is the sort of drivel that has been emptying movie theaters in recent years.

Professor Irwin Corey's inimitable craziness prevents his character, Joe Kaminsky, the heroine's screwball cabbie father, from sinking in purple prose and sentimental ooze. Corey embodies a vigorous kind of craziness that might have transformed the material into a farce about New York driving people loony, assuming Gardner had been inclined to follow his lead.