Rational moviegoers with both feet on the ground may find "Xanadu" strictly a Xana-don't, but there's something to be said for not having both feet on the ground and even something to be said for "Xanadu." Universal's trifling, featherweight musical, opening today at six area theaters, at least has a consistency of tone -- basically, both feet in the clouds -- and a bubbly, dippy dopiness that is vaguely related to charm.

What would be perfectly satisfactory as a 90-minute TV musical special appears awfully paltry as a 93-minute musical film, and yet "Xanadu" is distinguished among current movie releases is that it isn't genuinely of those tootsie-fruitsy confectionary fantasies that movies rarely indulge in any more, partly because when they did, the results could be agonizing pips like "One Touch of Venus" and "The Horn Blows at Midnight."

Maybe the success of "Heaven Can Wait" told the producers of "Xanadu" that the ticket-buying populace would sit still for a story about one of the nine muses of Zeus who comes alive from a mural and visits her mercurial benediction upon a struggling painter of record album covers.

The kid has "a dream" -- or rather, another dumb show-biz idea of a dream -- of opening a big nightclub, and the muse, along with a likable old hoofer, helps it come true. But then she finds herself falling prey to his mortal appeal and, well, you wouldn't believe, but you wouldn't particularly care, either.

It helps not that Olivia Newton-John, one of the clunkier sprites of all time, is musette and Michael Beck, the once-sultry menace of "The Warriors," is her leaden leading man, but it helps a great deal that Gene Kelly returns to the screen to play their adopted godfather. Kelly dances only a little, and certainly not in his old athletic style, but one number, a dream sequence (which, within the contest of this picture, is certainly a redundancy) half-casts the kind of spell Kelly threw in the romantic dances he did in movies like "Singin' in the Rain" and "An American in Paris."

He excuses himself from anything too elaborate by looking into a mirror and saying, "You're gettin' old, boy." In addition, the character he plays, once a big band leader and nightclub owner, reminisces about the swell joint he ran during the war and which the script calls "Danny McGuire's." This will warm the cockles of any heart partial to Kelly's 1944 musical classic "Cover Girl" with Rita Hayworth; Danny McGuire's was the name of the little club in Brooklyn run by Kelly and momentarily deserted by Hayworth when she hit the big time (The American Film Institute Theater will kick off a new film series with "Cover Girl" on Sept. 2).

It's a slim bit of resonance, but it contributes to the sweetness of the movie, a film so pleasantly boring that it would be impossible to take umbrage at it; compared to snide and slovenly concoctions like "Can't Stop the Music," it's the Hurricane Allen of fresh air.

The film seems particularly suitable for early adolescents whose capacity for appreciating romantic piffle is enormous. Though tecnically the film is a "rock musical," and sort of a closet roller-disco musical, many of the songs on the soundtrack are substantially attractive and tuneful numbers written and produced by Jeff Lynne and performed (off-screen) by Electric Light Orchestra, which has one of the most melodic traditions in modern pop.

Of course why a group like that would want to be associated with olivia Neuter-John remains a mystery, and why she was ever cast in another movie after her web-footed and knuckle-headed debut in "Grease" is a question that can only be answered in ruthless commercial terms. In "Xanadu" she looks ridiculous, she signs ineffectively, and she dances gawk-awkwardkly. About the only movie project she could possibly be suited for is a revival of the Gidget series -- "Gidget Goes Aussie."

Beck has more dignity to retain, but he doesn't exactly light up the screen, and one of his problems as a romantic lead is a cosmetic one; in profile, this kid's got a shnoz that looks like a just-plucked turnip. It says something about the chemistry between this forlorn twosome that the movie's only vaguely sensual moment is when cartoon versions of the two of them kiss during a brief and off-the-wall animated sequence.

Richard Christian Danus and Marc Reid Rubel, who wrote the screenplay, are to be admired only for having A Lot of Nerve, and director Robert Greenwald, incompetent at staging musical numbers, covers over his lack of basic filmmaking savvy with glitzy neon geegaws and dissolves that look like the closing of vertical venetian blinds -- admittedly cute tricks that don't exactly compromise the esthetic integrity of the movie, which has none.

The picture was photographed to no great advantage by Victor J. Kemper, but it's impossible to assess the look of it since it was screened at the Jenifer Cinema, where the art of projection continually hits new lows no one dared to think possible. When director John Landis saw his "Blues Brothers" movie at the Jenifer during a visit to Washington, he was stupefied and dismayed. Every movie shown there turns to mud on the screen.

"Xanadu" cannot possibly be described as a good movie, but it can be recommended to those who can tolerate large amounts of intravenous marzipan. The music is highly enjoyable -- though perhaps more so once one gets the record album home and isn't bothered with the story -- and the film so unerringly airy that it has a beneficent, tranquilizing, bemusing effect.

Then there is the Lord-Loveth-an-Awful-Movie factor, which says that any film in which a teen-age character says the line "Hey, Zeus?" can't be anything less than irresistible. On some daffy plane, "Xanadu" is; the negligibility of it all has allure in itself. "It must be frustrating to spend your time on things that don't matter to you," Newton-John says at one point, but this is not necessarily true of moviegoing. Hey Zeus -- send us another one like this some day.