Coward diary bows to mixed review; The Noel Coward Diaries, edited by Graham Payn and Sheridan Morley. Boston: Little, Brown & Co. 698 pp. $22.50.

The extraordinary theatrical career of Noel Coward meant that his diaries would have built-in interest, even though they warrant only mixed reviews. Their publication now, almost a decade after his passing, coincides with a Broadway season whose comedy hit has been ''Present Laughter,'' a play he wrote in 1939, shortly before the diaries begin.

When Coward portrayed Lord Mountbatten in ''In Which We Serve,'' he told his diary: ''Day spent doing scenes over and over again to try to eliminate Noel Coward mannerisms.'' The mannerisms can easily be imagined as the diaries run on.

They are not as searching or scintillating as might be hoped or expected. But through their almost 700 pages glitters a galaxy of names - Vivien (Leigh), Larry (Olivier), Willie (Maugham), Marlene (Dietrich), Johnny (Gielgud) - that keep the editors footnoting like mad and the stagestruck reader wallowing in stardust. As Coward records not only his own successes and failures but his responses to others, the book becomes a theatrical compendium of the times. For all the controversy over his private life, he is finally given a long-deferred knighthood, and the diaries end in 1969.

Carefully pruned for libel, the entries are often waspishly or sorrowfully critical of celebrated and not so celebrated figures. The worldly Coward seems to have a particular blind spot toward religion, belittling Christianity in general and echoing his familiar vituperation against the founder of this newspaper.

Yet he had his affections, too, particularly for the mother who started him on the stage and always stood by him. And, among the gossip, trivia, and scattering of four-letter words is a pride in the professional craftsmanship accompanying his talent to amuse.

While there are pithy lines, humor, and observations in the diaries, Coward obviously saved most of his wit for the stage. Here is a sense of some serious concerns behind the entertainer's mask. Here is the child of ''genteel poverty'' still worrying about money with homes in Britain, Jamaica, and Switzerland - but hobnobbing with the rich, whom he often finds boring, and the royal, whom he usually adores. A quintessential entry:

''It was all very family: only the King and Queen, Princess Margaret, all the Mountbattens and me. . . . After dinner I sang a few songs.''