GRANTED THAT pretense - "make-believe" - is the essence of theater, pretentiousness is its most abominable quality. Its presence signifies the lack of awareness of what is good, and why. A quality of being bad is to be ignorant of it.

On the current scale of pretentiousness there is Broadway's "American Buffalo," by David Mamet. Some presume to see its situation about three men in a junk shop as a parallel to Watergate. It is a pleasure to join the redoubtable John Simon in remarking that this is "an inept way of selling the play because if Mr. Mamet had really wanted to write about Watergate, I think he would have done it in a more pungent, unmistakable way." The key to the pretentiousness lies in the fact that those of Watergate pursued their schemes to disaster. Mamet's trio only talks.

"The Life and Times of Joseph Stalin" and "A Letter for Queen Victoria" illustrate how the avant-garde's Robert Wilson has anything to do with its assumed subject. Wilson, with his "Visual Semi-Surrealistic Effects," has no belief in the sacredness of words. Afraid of being sought out, the I-Want-Tp-Be-In set nods about depth and meanings, and some foundations go ape.

The French, not of the boulevards but of the intellectual, exportable variety, have a glorious knack for dramatic pretentiousness. After an early, impressive nibble of Sartre ("Les Mains Sales"), I encountered his essential phoniness in "The Respectful Prostitute," purporting to be about an American South - one he'd only read about. When, to finally fulfill an offer of the Eisenhower's stage, the D.C. Black Repertory COmpany chose Genet's "The Blacks," I was astounded that pretentiousness won out over what was a glowing opportunity for a homegrown work. The honesty and beauty of a Glenda Dickerson soul-musical was bypassed for a whoozy essay that had no meaning for the black audience it sough to attract. The We Happy Few showed up to applaud - and fell asleep.

"Odyssey," the Erich Segal-Mitch Leigh mash that evidently deteriorated, if such was possible, after its Kennedy Center bow, was bad in an obvious way. What appalled me most was its waste, the notion that Segal, having written the popular "Love Story" would be able to come up with a book for a musical about Homer because he is a Homeric scholar. Much the same whould happen later with such ideas as "1600 Pennsylvania Avenue" and "Rex," which, as soon as you heard about them, seemed phony. Millions were lost on these three alone, and obvious waste surely is nonrecognition of badness.

Finally, there are the T.S. examples: Twisted Shakespearare. This, to be sure, is no modern disease, for the 18th century arranged a happy ending for "King Lear" just as Jan Kott arranges to find in it Beckett's "Endgame." Twisted Shakespeare is harder to identify that at first seems. Distrustful as I am of it, I applauded Tyrone Guthrie's Edwardian "Troilus and Cressida" but had to gag over Jonathan Miller's juvenile "Hamlet," or the fashion of turnign "Henry V" into an antiwar poster, or the present distrust by the Folger Theater Group of the poetry in "A Midsummer Night's Dream." T.S. is not limited to Shakespeare twisting. It goes for all efforts to make revivals "relevant" by ignorant the proven, lasting good in them.