You don't have to be a baseball fan to enjoy ''Diamonds,'' at the Circle in the Square Downtown. This good-natured celebration of the American national pastime aims to please all comers and scores handily in the process.
''Diamonds'' is a multiple team effort. Its numerous contributors include a number of familiar names. Among them are Cy Coleman, Kander and Ebb, Comden and Green, von Tilzer and Norworth, and Abbott and Costello, represented by their legendary skit ''Who's on First.'' With director Harold Prince as head coach, the eclectic sampling of old and new material proves a lively if uneven frolic.
Certainly the most exotic parody of the occasion is Sean Kelly's hilarious ''Kasi Atta Batt,'' which turns out to be a Japanese Kabuki version, complete with lion dancer and samurai, of the lament known to untutored Western ears as ''Casey at the Bat.'' In a further gesture to baseball chic, Chip Zien assumes a ersatz French accent for a sing-along of ''Escorte Moi,'' a Gallic version of ''Take Me Out to the Ball Game.''
In more than 30 scenes and musical numbers, ''Diamonds'' covers its subject from Little League parents to the childhood of George Steinbrenner, the imperious owner of the New York Yankees. In one of its cinematic segments, the show's big screen lights up with filmed commentaries by such notables as Casey Stengel, US Rep. Tip O'Neill, Pearl Bailey, President Reagan, and Pope John Paul II. Very entertaining.
Musical moods are as varied as baseball weather. Singing ''Song for a Pinch Hitter,'' Susan Bigelow voices the plaint of a distaff grandstandee who learns the hard way that ERA stands for ''Earned Run Average'' rather than ''Equal Rights Amendment.'' In ''What You'd Call a Dream,'' Scott Holmes as a businessman in a three-piece suit fantasizes a la Walter Mitty.
Dick Latessa leads a dancing vocal ensemble in the gentle turn-of-the-century ragtime measures of ''Hundreds of Hats.'' Jackee Harry makes the rafters ring with the gospel-style ''He Threw Out the Ball,'' and pays stirring tribute to the long honor roll of once-segregated black baseball players in ''Stay in Your Own Back Yard.'' Versatile Harry Riley accompanies himself on guitar in ''1919, '' Jim Wann's country-style ballad about the Chicago Black Sox scandal. The excellent starting lineup includes Loni Ackerman, Nestor Serrano, and young Dwayne Markee and Wade Raley, who (at alternate performances) take care of junior assignments.
Tony Straiges has converted the Circle in the Square arena into a ministadium replete with regulation seats. The revue was costumed by Judith Dolan and lighted by Ken Billington. It's no forced play on words to say that ''Diamonds'' is a small gem of a show.