Jerry Herman's biographical musical about the romance between Mack Sennett and Mabel Normand ought to have been a winner.  It has a terrific score -- perhaps Herman's finest -- and its leading characters, the pioneer of screen comedy and his leading lady, were two of the biggest names in silent movies.  Besides, Sennett was famous for custard pie fights, bathing belles and the slapstick lunacy of the Keystone Cops, so the subject has plenty of potential for colour and movement.     But the show flopped in 1974, running only 65 performances despite Robert Preston and Bernadette Peters in the eponymous roles.  Torvill and Dean's use of its catchy overture for a skating routine at the 1984 winter Olympics gave the music wide popularity, and various revised productions were mounted overseas.  But still it failed to measure up as a hit.     It certainly isn't the fault of the music.  Herman is always melodic and uncomplicated, if prey to the odd cliche and a predilection for the archetypal show tune in 2/4 time.  For Mack and Mabel, he furnishes a succession of irrepressible songs: the anti-romantic I Won't Send Roses, one of his choicest ballads; Movies Were Movies, a rousing paean to the pioneering days of movie-making; Tap Your Troubles Away, a parody of all relentlessly optimistic anthems; and the snappy Wherever He Ain't and the torchy Time Heals Everything, just the sort of showy numbers to bring out the best in any leading lady.     These songs and the rest of the score are put across wonderfully well by the stellar cast in the Production Company's concert staging.  John Diedrich and Caroline O'Connor, as Mack and Mabel, have all the requisite star presence and the big voices to make every musical post a winner, and Leonie Page (as the hoofer, Lottie Ames) is right up with them in her solo spots.     They are supported by a strong ensemble under Jo-Anne Robinson's sharp, sure direction, in which Robyn Arthur (Ella), Jonathan Mill (Wally) and Troy Sussman and Mark Dickinson (a pair of studio money men) stand out.  Leanne White's choreography is executed with great precision in the girls' tap number, and the orchestral playing by members of the State Orchestra of Victoria, conducted by Guy Simpson, is a total delight, specially the brass and percussion.     It's Michael Stewart's dialogue that is the show's weakness.  As hard as Diedrich and O'Connor work to make their characters come alive, they are held back by a fragmented book that gives no depth to Sennett's and Normand's troubled love affair.     Even for a musical, key scenes are coy and contrived where they should pack some sort of emotional punch.  But if you go to a musical to tap your toes, Mack and Mabel is for you.