s might be expected, ''Beverly! Her Farewell Performance'' is more laughter than tears, more celebration than farewell. The twohour program, to be broadcast tonight over the Public Broadcasting Service, is a telecast of an Oct. 27 gala at the New York State Theater, the last of several national farewell appearances given by the much-loved Beverly Sills, now director of the New York City Opera. On hand are friends and colleagues from the concert stage, opera, ballet, theater and television, and the air is rich with affection.

The gala opens, as it should, with the camera sweeping across the City Opera singers, ranged about the stage for a wacky version of the second act of ''Die Fledermaus.'' In the role of Prince Orlofsky, Kitty Carlisle Hart gives fair warning to purists as she sings ''Chacun a Son Gout.'' The sight of Leontyne Price and Renata Scotto spending those vocal riches on, respectively, a labored rendition of ''What I Did for Love'' and a treacly ''Over the Rainbow,'' or James Galway, however charmingly, wasting that flute on ''Danny Boy,'' may not be to everyone's taste. Why not unleash Ethel Merman on ''Vissi d'Arte''?

But this, after all, is a party, and it is given in honor of a great singer who will probably always be ''Bubbles'' to those she has welcomed, with her endearingly practical air, to the sometimes forbidding world of opera. And the show kids itself. ''Oh, no, not another one,'' the major domo, James Billings, groans as he introduces yet another pop-singing opera star, Sherrill Milnes, who belts out an exultant ''Maria.'' Sly ad libbing takes Dinah Shore's ''One More Time'' beyond doughy homage.

And there are many unforgettable moments in this party. Chief among them is Mary Martin in a breezy, gilt-edged performance of ''My Heart Belongs to Daddy.'' Donald Gramm brings his customary urbane conviction to ''I Want What I Want When I Want It,'' doctored for the occasion. Miss Sills and Carol Burnett, the program's master of ceremonies, get together for their funny and touching ''Sufferin' Blues Medley.'' Miss Merman lets loose with ''There's No Business Like Show Business'' one more time - there can never be too many - and Bobby Short offers a nicely apposite ''Nashville Nightingale.''

Julius Rudel, conducting the versatile City Opera orchestra, proves to be a bluesman par excellence, and Walter Cronkite leads Miss Sills through a remarkably abandoned cross-stage waltz. Throughout, there are tantalizing peeps at a celebrity audience in best bibs and tuckers.

James Lipton produced and directed the gala sequence. Music staging and choreography were by Ron Field, and special lyrics were created by Betty Comden and Adolph Green and Martin Charnin. Produced by the Lincoln Center for the Performing Arts, ''Beverly! Her Farewell Performance'' was financed by Exxon, the Andrew W. Mellon Foundation, the National Endowment for the Arts and the Corporation for Public Broadcasting.