The Reagan administration dismissed Nicaragua's latest diplomatic overtures as insignificant and containing "a lot of air," even while maintaining that this sign of movement in Managua is evidence that U.S. policy is working.

Secretary of State George P. Shultz joined Vice President Bush and other spokesmen in a skeptical reaction to Nicaragua's announcement Wednesday night that 100 Cuban military instructors will be sent home and that no Soviet MiGs or other new jet fighters will be sought for the "indefinite" future.

But Shultz also said he is willing to explore the proposals with Nicaraguan President Daniel Ortega in Montevideo, Uruguay, where both men will be today for the inauguration of a new Uruguayan president. Washington Post staff writer John M. Goshko reported from Shultz's traveling party that Shultz said "I'll listen carefully" to Ortega if a meeting can be arranged.

Speaking to reporters on arrival last night in Montevideo, Ortega said he was willing to meet with Shultz. In Managua, Nicaraguan Vice President Sergio Ramirez said Nicaragua also was taking steps to resolve a diplomatic dispute with neighboring Costa Rica in hopes of stimulating stalled regional peace talks. Details on Page A16.

On Capitol Hill, increasingly the focus of a high-profile political battle over U.S. "covert" funding for antigovernment guerrillas in Nicaragua, congressional leaders reacted cautiously to Ortega's invitation to send a delegation of lawmakers for an on-the-spot study of "the defensive character of our country's armed forces and defense systems."

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), a leading critic of administration policy on Central America, replied to Ortega that he is forwarding the invitation to Shultz and the executive branch because "the president is responsible for the conduct of my country's relations with other nations."

O'Neill also referred the invitation to the House Foreign Affairs and Intelligence committees. Chairman Dante B. Fascell (D-Fla.) of the Foreign Affairs Committee said through a spokesman that he does not believe that such a mission to Managua would be a good idea "at this time, when the U.S. and Nicaraguan governments are not holding talks."

The first and most scornful reaction to Ortega's announcement came from White House spokesman Larry Speakes, who said that "there is nothing in his peace proposals" but added that Ortega is "making noises down there that he hasn't made in the last couple of years." To that extent, Speakes said, "Our policy is succeeding."

In a later briefing, Speakes said of Ortega, "He's not putting much out there. There's a lot of air in there."

Speakes said Ortega "has launched what appears to be a fairly sophisticated offensive here to influence the United States Congress."

Bush, addressing the Council on Foreign Affairs in Austin, Tex., delivered a lengthy denunciation of "the communist rulers" of Nicaragua and appealed to Congress "to release a few dollars" to make possible "technical, material and financial support" for the guerrillas.

In what was described as the most authoritative U.S. reaction to Ortega, Bush said, "Something must be working when changes like this -- if indeed they are changes -- occur." He added, "We would surely welcome genuine Nicaraguan interest in peace," and called for "specific, concrete and far-reaching actions that would show their good-faith interest in peace."

The statements from Managua so far "do not appear to represent significant moves," Bush said. He described Nicaragua's announcement that 100 Cuban military advisers will go home as "offering to remove 1 percent of the Cuban presence." State Department spokesman Ed Djerejian said the administration estimates the number of Cubans in Nicaragua as about 7,500, of whom 2,500 to 3,500 are "military security personnel." The Soviet Union, he said, has "over 200 people there."

Bush described Nicaragua's declaration on defensive weapons as a "pause in their import of arms which they acknowledge could not be absorbed at this time."

As broadcast by Nicaraguan state radio Wednesday night, Ortega said that "we have decided on the declaration of an indefinite moratorium in the acquisition of new arms systems, as well as those interceptor aircraft required for the completion of the country's existing antiaircraft system."

In the past, the Reagan administration has expressed serious concern -- and warnings -- about the possibility that Nicaragua would acquire MiG fighters.

Bush said, "It is relevant to ask why they bother. Is it because Nicaraguan young men are refusing to serve in the army out of revulsion at their government's policies and are joining the resistance? Is it because of the outrage being expressed by the church in Nicaragua? Is it because of the collapse of their economy under the weight of Sandinista militarism and corruption? Is it because their people see what is going on in neighboring countries and want it for themselves?

"Perhaps it is because of all these reasons. Perhaps it is because the Scoop Jackson [U.S. aid] plan is working. Perhaps it is because the struggling freedom fighters appeal to the people of Nicaragua."

Bush seemed to go out of his way to highlight possible trouble between Nicaragua and its southern neighbor, Costa Rica, quoting Nicaraguan Interior Minister Tomas Borge as calling Costa Rica "the dessert" in the process of Central American reunification. "What he meant was that tiny Costa Rica, a longstanding stable democracy that keeps no army, would be completely vulnerable to armed aggression -- a piece of cake, as it were."

Bush's office said the quotation from Borge was taken from a Dec. 2, 1983, Wall Street Journal editorial-page article by Huber Matos Jr., information director of an anti-Castro group in Costa Rica. Matos, whose father was imprisoned by Cuban President Fidel Castro for more than two decades, gave no source for his quotation.

Bush noted in his address that some people doubted whether the Sandinista leaders in Nicaragua are Marxist-Leninists. As a dramatic high point, he held up Nicaraguan postage stamps bearing a picture of Karl Marx. "I would simply say, here are the stamps," he told the group.

Bush set forth four U.S. demands of Nicaragua that were also mentioned yesterday by Shultz and other spokesmen. As stated by Bush, they were "that the Sandinistas stop exporting subversion to their neighbors, that they reduce their bloated military to restore regional balance, that they sever military ties with Cuba and the Soviet bloc, and begin to honor their promises to the Organization of American States to create a democratic, pluralistic system." Bush said the last point was "perhaps most fundamental."