For some mysterious reason, Jim Henson and his associates have arranged an unfortunate film debut for the Muppets. "The Muppet Movie," opening today at the Jenifer, suffers from a desire to be ingratiating in all the wrong ways.

It's as if the opportunity to make a film proved such a dream-come-true that the Muppet Brain Trust went into a mellow funk. Instead of charging onto the big screen with a rousing, uninhibited comedic fanfare, the Muppets meander through a sluggish plot trailing fragile little clouds of wistfulness.

The movie - in which the Muppets migrate to Hollywood - reverses the priorities of television's "Muppet Show," which concentrates on gratuitous jokes, relieved every so often by an insipid ballad.

Like clowns longing to play Hamlet, Henson and his colleagues may have undervalued their truly original and endearing comic impulses. Occasionally on the TV show, Kermit the Frog is exploited for his plaintiveness - a negligible failing on television but a liability in "The Muppet Movie," where sentimental notes have been allowed to suffocate the gags.

Jerry Juhl and Jack Burns, the chief writers of "The Muppet Show," have abandoned its backstage revue format in favor of a plot retarded by a retrograde tempo and the obligation to make room for bit appearances by more than a dozen "guest stars."

The movie-within-a-movie scenario shows how Kermit, Fozzie Bear, Miss Piggy, The Great Gonzo and other members of the Muppet troupe first met and Followed a Dream to eventual fulfillment on the sound stages of Hollywood. After a fairly promising framing sequence, in which the Muppets are assembled for a rowdy private screening of their cinematic work-in-progress, the story proper begins in Kermit's native swamp, where Kermit is discovered warbling a simpery, overorchestrated ballad about rainbows.

The first guest star, Dom DeLuise as a lost talent agent, inspires the dreams of Hollywood glory. Beguiled by Kermit's singing, the agent suggests that Kermit might "make millions of people happy" if he hopped out to The Coast. Kermit finds the suggestion irresistible on purely humanitarian grounds and hits the road, encountering the other Muppets on his travels. Along the way he is menaced by a human villian - Charles Durning as a fast-food tycoon - who craves Kermit as the symbol for his "chicken-fried frog's legs" franchises.

Surprisingly, the Muppets did not create insurmountable technical or esthetic problems for the filmmakers. The puppet figures are deftly integrated with miniature sets, full-scale sets, live actors and exterior locations. Some of the pictorial illusions are enchanting, especially an exterior sequence of Kermit peddling merrily along on a bicycle. It appears that ingenious scenic design and sophisticated remote control devices will make it possible to introduce the Muppets into any conceivable setting. Their charm isn't confined to a small scale image.

The letdown originates where you lease expect it - in the script. Even if a sustained plot was necessary, it should not have wasted the advantage secured by "The Muppet Show," on which the characters are already established personalities and performers. The movie, pretending to recall a period before the Muppets became stars, gets bogged down in insignificant show biz pre-history.

Surely "The Muppet Movie" has created greater expectations of a good time than most of this summer's new releases. And it might have fulfilled them if it had caught up with the pace, tone and consistency of "The Muppet Show."

The TV series also uses guest celebrities, but they're rationed one to a segment and integrated into the Muppets' format. The laborious pace of the movie lags even more each time a guest star must be wedged into the continuity. Some performers are short and sweet, like Madeline Kahn as a B-girl saying "Hello, sailor, buy me a drink" to Kermit at the bar of the El Sleezo Cafe. Others are obtrusive and sour, particularly Steve Martin in a bit as a sneering waiter. All of them, tolerable or intolerable, happen to be expendable.

It seems symbolic of the script's coughing-and-sputtering lack of progress when the car transporting the Muppets ever westward breaks down in the desert. Mooning around the campfire, Kermit gets speeches like "I guess we blew it, huh Gonzo?" and "What do I know about Hollywood anyway?" and "I brought us out here in the middle of nowhere when all I wanted was to make millions of people happy."

They are good questions. The Muppets have done all right without Hollywood so far. In fact, they even made it to the movies without Hollywood, since "The Muppet Movie" was produced through the auspices of the British entertainment czar Lord Lew Grade, who also packages "The Muppet Show." Perhaps it's this fixation on Hollywood as an end in itself and an ultimate symbol of success that prevents the Muppets from hitting their cutomary zany stride in "The Muppet Movie."

Curiously, once the Muppets finally reach the promised land of Hollywood, they don't have to do anything to make the big time.A mogul called Lew Lord, embodied by the increasingly fat and fatuous Orson Welles, hires the gang on impulse (or inspired hunch if you prefer), without waiting for an audition, making their success look as arbitrary and baffling as that of some starlets.

Will kids devoted to "The Muppet Show" be disappointed by a Muppet movie that often feels as listless as a Benji movie? I can't speak for multitudes, but my oldest daughters, an 8-year-old and a 5-year-old who never miss the television show, sat through the movie in near silence and progressive fidgetiness. They didn't dislike it, but the active enjoyment they derive from "The Muppet Show" was rarely stimulated.

The next time why not let the Muppets impose their peculiar genius on the film medium? Doing it the other way around is simply the wrong way of going about it. CAPTION: Picture 1, Kermit the Frog in "The Muppet Movie"; Picture 2, Kermit, Steve Martin and Miss Piggy