FOR an institution routinely disparaged as ineffectual (or as the new US ambassador, Vernon Walters, puts it, a ``measured disappointment''), the United Nations is certainly attracting a remarkable pageant of statesmen to celebrate its 40th year. Today the General Assembly debate begins. By Oct. 24, the anniversary of the UN Charter signing in 1945 in San Francisco, nearly 100 heads of state or government will have appeared for the full motorcade, photo, and rhetorical treatment at the UN's Manhattan headquarters: King Juan Carlos of Spain; Generals Jaruzelski of Poland and Zia of Pakistan; Colonel Qaddafi of Libya (his first US trek); Prime Ministers Thatcher of Britain and Gandhi of India, Nakasone of Japan and Ziyang of China; Presidents Muba r- ak of Egypt and (likely) Mitterrand of France; Chancellor Kohl of West Germany. Latin American leaders will appear in such abundance that a mini-summit is planned to discuss drug traffic, debt, and Central American issues. The most missed may be the new Soviet leader, Gorbachev, who will meet a month later in the maxi-summit with President Reagan.
Only the cynical could observe that such an unprecedented bazaar of world leaders would assemble to disparage the world organization of 159 member states.
The measure of disappointment in the world body noted by Ambassador Walters should be examined. In the three war years from 1941 to 1944, United States public opinion shifted sharply in favor of joining a new world organization. At the time of the charter's signing, the public had strong doubts about the organization's effectiveness in key tests like preventing future wars. Since then, the American public's rating of United Nations performance has, overall, slipped: from about a 2-to-1 positive rating i n the early 1950s to a 5-to-3 negative low point in 1980, before recovering to an almost even rating now.
Public commitment to the UN as an institution, however, has remained resolute over the years -- some 8 in 10 firmly supporting membership, while 1 in 9 or 10 would pull out.
True, there are divisions over the amount of influence the third-world nations have in the United Nations today, and complaints that United States influence is too little. This partly explains support for the proposal to trim US financial support of the UN from 25 percent to 20 percent of the body's budget.
Nonetheless, even in the habitually idealistic United States, general expectations for the UN have always been more realistically tempered than its critics would suggest, while backing for the institution has withstood frustrations over impotence in trouble spots like the Middle East, South Africa, and Central America.