LAST WEEK President Reagan casually endorsed an American diplomat's taunting suggestion, one offered in a spirit of facetiousness, that the U.N. feel free to move out of New York if its members were so disposed. The Senate no less casually accepted an instant floor amendment to take a big whack out of American payments to the world body. All this was done as Mr. Reagan hoped for a sympathetic reception of the arms control message he intends to deliver to the General Assembly on Monday, and as he contemplates dropping the Lebanon problem back into the U.N.'s lap.

Most people will probably acknowledge, if they think about it, that the U.N. is an easy scapegoat for a variety of frustrations not of its own making. Most people, again, will grant that the U.N.'s peacekeeping role is, if imperfect, indispensable, and that it performs other useful services. But the organization remains a source of continuing disappointment, most of all to those who came to it believing in the dream of liberal internationalism that it embodied at its founding. The failure of member nations to tame their more rampant and selfish sovereign impulses comes through in regular abuse of its forums and procedures for purposes that demonstrably do not serve the common good.

Fair criticism of the goings-on at the U.N. sometimes gives way, however, to simple U.N.-bashing. This seems to be what happened in the Senate dues vote, which arose by chance just as the matter of the U.N.'s New York tenure was in the news. Many observers, close and distant, have long been dismayed by the U.N.'s financial and administrative practices. Only a few days ago, the secretary general himself said that much criticism of the U.N.'s administration as "inflated, politicized or extravagant" is justified. A careful approach in the Senate, framed to bolster the U.N.'s own reform impulses, could have helped. But many other considerations, including resentment over some recent Security Council votes that did not go exactly the American way, intruded on the Senate's snap deliberations.

It falls to the Reagan administration to repair the damage, which is not mortal, as best it can. It has to cultivate an atmosphere in which American initiatives at the U.N. can be treated on their merits, and it has to make the United States a working partner in the world body's internal reform. The president would simplify his own task if he avoided occasions to play to the considerable gallery that only dimly understands that the U.N., for all its real and imagined shortcomings, can still serve some American interests well.