FROM THE DAY it opened, under inexplicably heavy fire, the Hirshhorn Museum has brought some mighty fine art to Washington, the works of Saul Steinberg - on display through Nov. 27 - being not only among the best of the best, but also peculiarly appropriate. Like Washington, Mr. Steinberg specializes in plans, seals, monuments, certificates, diplomas, fingerprints, tools, forts and wars. If his official documents are unintelligible, there is no question of their importance. And the florid signatures in his works are clearly of enormous authority, even though they are impossible to read.

He is a very funny man. He also draws exceptionally well - something one doesn't appreciate from only seeing Steinberg cartoons in The New Yorker. There is, for example, the "Hostess Mask" of a well-planned face, which looks like an equally terrified and terrifying Angela Lansbury. There is "The Tree" - a too-beautiful work of calendar art that, for all its deliberately perfect tree and gorgeous sky, is, in fact, beautiful.

Of course, "The Tree" has a couple of Steinberg seals of approval stamped on it, which allow the artist to comment at once favorably on his own work and unfavorably on the world of official judgment. If there is one thing Mr. Steinberg is not concerned about, it's official judgment. Even his "Six Terrorists." three of whom look like Felix the Cat, are not condemned by anything beyond their appearance.

What Mr. Steinberg proves in this exhibit (as if he were out to) is that the comic imagination is no less critical or difficult than the tragic; it simply seeks a different initial response. It is perfectly acceptable to laugh out loud at a Steinberg, even the most harsh and blatant. What else is one to do upon confronting an idea of "Evolution," which starts out with a baby, moves up through a dog-headed man (or a man-bodied dog), and winds up as a snail? What else is Washington to do as it studies the combination of illegible notes, postmarks, pie charts, diagrams, marshes, alligators, canoeists, and - of course - the official seals that make up "Government Regulations on Landscape"?