MSTISLAV Rostropovich, it comes as no surprise, has joined the ranks of the superstars. He's got the artistic stature, the personal magnetism and the added, faintly exotic, appeal of a Russian who has declared his independence. In Washington, Rostropovich already commands an almost automatically manic response.

Superstars, however, tend to generate superstar complexes among their admirers. The peril is lapsed or distorted judgment - a worshipful acceptance of good and bad alike, a myopic incapacity to detect or acknowledge shortcomings. In the long run, such clouded discernment is beneficial to no one, neither the artist nor his audience.

It's not my aim to belittle what may become a fruitful love affair between a music-hungry Washington public and its new hero. I do suggest that, at this early stage of the relationship, a bit of ritardando might be prudent.

As a conductor, after all, Rostropovich is largely an untested and unknown quantity thus far. It may be heretical to say so in view of the nearly universal acclaim he has garnered, but there are also those, such as myself, who think that even his cello playing is not invariably above reproach. And then there is the matter of the programs he has outlined for his first National Symphony season, which are - let's face it - appalling.

Rostropovich has, without any question, the secret of turning audiences on. As a direct consequence, he's a very hot number at the box-office, a fact that certainly has not been lost on the National Symphony board of directors which appointed him to his upcoming term as director.

His credentials as a performing artist of distinction are also impervious to challenge. His cello repertoire is vast and varied, and it has been especially enriched through scores engendered by his close personal relationship with some outstanding composers, including Prokofiev, Bernstein and Britten. He's a brilliant technician and colorist. His interpretive authority in certain portions of the repertoire - the Russians, for instance - would be hard to impugn.

Even the scanty evidence of his orchestral conducting in Washington, however, suggests that Rostropovich may have his vulnerable sides as well. Some of his interpretive notions that strike one as arbitrary may represent no more than a healthy unorthodoxy, or personal idiosyncracy of an interesting kind. My own impression, however, is that Rostropovich, both as a conductor and a cellist, frequently lets his drive for impassioned expression get the better of his sense of order, style or euphony.

Because he is a string player, he can get away with pushing the strings to exhartatory extremes without greatly impairing their tonal sheen. The winds and brasses, however, have sometimes sounded almost coarse under his baton, just as the corresponding sections of Soviet orchestras do in passages of overdrawn climax. Perhaps Tchaikovsky or Berlioz wouldn't suffer too much from such handling, but the classical repertoire can't take it and stay very classical at the same time.

As a cellist, too, Rostropovich has his weaknesses, in this observer's view. From where I sat in the Kennedy Center Concert Hall last Monday (toward orchestra rear, left side), for example, in the Brahms E Minor Sonata he performed the quality was less than appetizing - buzzy or gruff too much of the time. Some of this can be ascribed to hall acoustics, which are considerably less ingratiating than they're sometimes cracked up to be, particularly for recitals or chamber music. It also might have helped to have lifted the piano lid further than the first peg for this muscular opus; pianist Samuel Sanders often had to pound to project his part adequately.

Apart from factors he was unable to control, however, Rostropovich's own contributions to the Brahms also seemed flawed. The melodic statements of the opening movement, for instance, were all run together, with scarcely any lifting of the bow to indicate phrase structure - or so it sounded. The breakneck tempo of the final movement, as another example, ruled out any possibility of linear clarity, given the knotty fugal texture, so much of it in the growly lower registers, which prevails through most of it. The Bach C Major Suite that followed was marred, among other things, by an excess of legato, softening and obscuring the sharp rhythmic articulation of the Baroque idiom. Even the rachmaninoff and Prokofiev pieces of the second half, which were magnificently set forth for the most part, was blemished here and there by forced, snarling sounds or bent pitches.

I'm not trying to suggest that Rostropovich doesn't deserve his reputation as an extraordinary cellist, or that he is less than gifted as a conductor - only that we would be wise to be on guard against letting our awe become mere reflex. Rostropovich will be taking over the National at a particularly crucial juncture.

Antal Dorati, his predecessor, will be leaving a legacy of three momentous accomplishments - a thoroughly rejuventated morale among the players and audience; the start of a really unified ensembled approach; and a superbly progressive revision of the previous stale, timid programming policy. The signs for continued high morale are good; Rostropovich seems to inspire a mixture of respect, affection and enthusiasm in his players, according to reports. In the other two categories, however, he will have his work cut out for him just to insure moving ahead, rather than backward.

The just announced slate of programs for the 1977-78 season is scarcely encouraging. Consider that aside from a single program of American music - to be conducted by Aaron Copland - the three other program slots assigned to an American composer are all given over to Leonard Bernstein at one fell swoop. And apart from these pieces, the only other music that could be considered remotely recent will be the Ginastera Cello Concerto and a commissioned work by Henri Dutilleux. The rest of the fare is so overloaded with chestnuts you could fill a Christmas stocking with them.

Dorati, we had thought, had finally and permanently delivered us from an era of provincial ultraconservatism. Unless Rostropovich is moved to rethink his premises, however, we are liable to be plunged right back in. And the way to orchestral greatness assuredly doesn't lie down that path.