'Jimmy Dean' crumbles as; 'Sally and Marsha' generates warm and affecting comedy; Sally and Marsha; Comedy by Sybille Pearson. Directed by Lynne Meadow.

''Sally and Marsha'' traces the friendship of an odd couple - distaff side - from awkward beginnings to touching farewells. In between, playwright Sybille Pearson gently and humorously explores the anatomy of a relationship in the often isolating world of New York City apartment-house living. It is a daytime relationship, conducted while husbands are at work and children at school.

Bernadette Peters plays Sally from South Dakota - pious, intensely domestic country mouse of this Big Apple encounter. Christine Baranski is Sally's across-the-hall neighbor, the profane city mouse - glib, neurotic, sardonic Marsha, whose dead-pan ironies and put-ons frequently sail right over the head of the recently arrived innocent. Loneliness forms the common bond of the two 30 -year-old matrons, who have lost their illusions without quite abandoning their hopes.

Sally's ingenious overtures and urgent hospitality at first disconcert and then disarm the offhand but vulnerable Marsha. In the seven months of the play's duration, the caring Sally becomes briefly the cared-for as the naive South Dakotan and the hard-bitten New Yorker teach and learn from each other.

In the course of the minimal plot, Sally becomes pregnant while the impatient Marsha resumes her education. The changed situations breaks the women's routine and leads to tensions that threaten the shared confidences of the relationship. Reconciliation owes as much to Marsha's rough honesty as to Sally's gentler heart wisdom. Under Lynne Meadow's responsive direction, the comedy's interplay of emotions is beautifully realized by the team of Peters and Baranski. This is not a singles match but a singular matching. The audience is the winner.

Occasionally, Miss Pearson seems to be overstretching her familiar material to touch yet another emotional facet. Slightness approaches frailty. Yet ''Sally and Marsha'' remains credible. It is a warm and affecting work, full of humane concern for its heroines' all-too-human concerns. Scenic designer Stuart Wurtzel has accommodated the conversation piece in an amusing variant of the West Side Manhattan apartment setting. The attractive production at the Manhattan Theater Club was costumed by Patricia McGourty and lighted by Marc B. Weiss.

Come Back to the 5 & Dime Jimmy Dean, Jimmy Dean

Starring Sandy Dennis, Cher, Karen Black. Comedy by Ed Graczyk. Directed by Robert Altman.

New York The stuff of dreams crumbles into stuff and nonsense in the cumbersomely titled new comedy fantasy at the Martin Beck Theater. ''Come to the 5 & Dime Jimmy Dean, Jimmy Dean'' has assembled the kind of high-voltage talent that creates its own glitter. Cher, the superstar of TV, recordings, Las Vegas, and countless magazine covers, makes her Broadway debut in the company of award-winning actresses Sandy Dennis and Karen Black. The director is Robert Altman of ''M*A*S*H,'' ''McCabe and Mrs. Miller,'' ''Nashville,'' and other films, plus Off Broadway's recent ''2 by South.''

In show business, however, high voltage doesn't guarantee electric entertainment. ''Jimmy Dean'' achieves no more glow than a 40-watt bulb. It is at the same time preposterous and negligible.

Playwright Ed Graczyk has imagined what might happen at the 20th reunion of the Disciples of Jimmy Dean in a Texas town near where ''Giant'' was filmed. The fan club's central twosome carry the memories and scars of that dizzying moment of glory. Mona (Miss Dennis), who played an extra in the movie, even claims that Dean fathered her illegitimate son. In the course of their reunion, the celebrants are joined by a mysterious stranger (Miss Black), whom the others nevertheless vaguely recognize.

''Jimmy Dean'' grows steadily more complicated and less diverting as Mr. Graczyk unfolds his store of unsurprising revelations. Needless to say, nothing is what it seems to be in the mirrored world of the five-and-dime as the action fluctuates between 1975 and 1955. David Gropman's elaborate double setting for the small-town Woolworth's, and Paul Gallo's lighting, do what they can to assist the illusion of time changes. But they ultimately compound the general confusion.

The audience is left with one of those truth-telling orgies featuring the kinds of sordid schlock that fill the columns of the more sensational weeklies. All things considered, the players make the best of the opportunities provided them. They do not seem to have received much help from Mr. Altman's rudimentary staging. Cher is likably forthright as the vulgar and promiscuous Sissy. Miss Dennis does an elaborate Sandy Dennis impression as Mona, and Miss Black postures effectively as the enigmatic Joanne. Sudie Bond, Marta Heflin, and Kathy Bates conscientiously represent the kinds of Texas stereotypes intended to help flavor plays set in the Lone Star State.Lullaby and GoodnightA ''Musical Romance'' written, composed, and directed by Elizabeth Swados.

Multi-talented Elizabeth Swados and the front-line New York Shakespeare Festival have lapsed regrettably with ''Lullaby and Goodnight.'' At her best, Miss Swados has created such stage works as ''Nightclub Cantata,'' ''Runaways,'' and ''The Haggadah.'' She has written books and composed scores for every media. In this context her ironically subtitled ''musical romance'' is a disappointment.

''Lullaby and Goodnight'' trivializes the tragic problem of prostitution with a garish, cabaret-style musical show. The energetic, morally muddled piece concerns Lullaby (Jossie de Guzman), a battered wife who escapes to New York. At the Port Authority Bus Terminal she is accosted by a pimp named Snow (Larry Marshall), who initially befriends her and ultimately brutalizes and degrades her. With the prostitutes' revolt led by Lullaby and Snow's final, abject surrender, ''Lullaby and Goodnight'' throws its little store of credibility to the winds.

There are indications that Miss Swados intended ''Lullaby and Goodnight'' in part as feminist statement and social commentary. What emerges on the stage of the Public/Newman Theater is a relatively slick, sentimentally tarnished piece of sensationalism. Musically interesting at times, it is well performed by the two principals, the singing-dancing ensemble, and onstage orchestra.