EDDIE'S CATCHY TUNES, a new musical by Tim Grundmann. Directed by Ken Bloom; sets by Russell Metheny; choreography by Anne Day; costumes by Peter J. Zakutanski and Ingrid Crepeau; lighting by Tomm Tomlinson. With Donald F. Campbell, Tonette Hartmann, Dana Vance, Jan Frederick Shiffman, Barbara Rappaport, A. David Jonhson, T. G. Finkbinder, Tanis Roach and Agnes Cummings. At New Playwrights' Theatre, 1742 Church St, NW, Wednesday through Sunday evenings until May 20.

For reasons not to be explained here, the sheet music to "Only a Mermaid," the show-within-a-show in Tim Grundmann's new backstage musical, is misappropriated by an evil goddness and later reappears pasted to a papier mache donkey.

The donkey, what's more, gets delivered to the cast-within-a-cast and the orchestra-within-an-orchestra just at curtain-within-a-curtain time. The show, needless to say, must go on Mickey Rooney would be proud of the way these game troupers tear that donkey to shreds and learn their songs in two minutes flat.

But the critics in the show are typically uncaring. "Last night," writes one, "the State of New York reinstated capital punishment in thr form of a musical comedy."

The utterly pointless yet strangely cleansing work from whence all this inspired dizziness springs is called "Eddie's Catchy Tunes," and it is to be found at the newly reopened New Playwrights' Theatre on Church Street.

To the extent that "Eddie" has a discernible plot, it involves an incompetent songwriter commissioned to write a Broadway musical who gets heavenly assistance from Calliope, the muse of music. Calliope drops from the clouds to lend a hand despite her complaint that she has four other Broadway openings in the same week and "Stephen Sondheim on my back."

This is a premise that offers author-composer Grundmann a broad license to write dumb. But he seems to have exercised that license as liberally on "Eddie," the show without as on "Mermaid," the show within. His boldest moments are surrounded with devices of staggering unoriginality-one whole character (the one played by T. G. Finkbinder" has been carved out of the hide of Mel Brooks' "The Producers," for example.

A crisper, brasher production might have helped matters, but it looks as though this company would be more at ease doing a straight play than a musical.

Donald F. Campbell, good actor though he may be, ought to reach Olympian heights of gawky, manic oversinecerity as Eddie, the songwriter with a lifelong case of writer's block; instead, stifled by a faltering singing voice, he and Eddie end up somewhere in the low Poconos.

Among the smaller roles, Jan Frederick Shiffman is oozingly hysterical as a leading man, and Tonette Hartman, who portrays a goddess, sings like one. Otherwise, the cast squanders far too many of what ought to be the show's funniest and most melodious moments.

Ingrid Crepeau's and Peter J. Zakutansky's costumes, though, make shabbiness a comic virtue. (During a bang-up number called "The Happy Fisherman," Shiffman wears a cardboard rowboat that dangles preposterously from his suspenders.) And Russell Metheny's amibitious, all-pink scenery is equally therapeutic.

If you're looking for consistency, look elsewhere. But if you need a loose evening to forget a hard day, "Eddie's Catchy Tunes" may be anarchy just up your alley.