The United States remains irritated that Mexico has stuck to its pro-Nicaragua policies in Central America and to its Third World orientation at the United Nations, U.S. officials said this week as they assessed prospects for the Jan. 3 summit meeting between Presidents Reagan and Miguel de la Madrid.

The U.S. government considers that Mexico has made modest improvements in its Central America policy this year but has not gone nearly far enough to satisfy Washington, the officials said.

"What we're looking for is, are there subtle changes in their position that allow us to move closer together?" said a senior U.S. official who is familiar with preparations for the presidents' meeting. "We have different positions on Central America. We'll ask de la Madrid's position and see if it's changed at all," he added.

The U.S. officials' comments appeared designed to pressure Mexico to alter its foreign policy at a time when the de la Madrid government is seeking U.S. assistance in handling Mexico's $96 billion foreign debt. Mexico wants official U.S. support for its plans to borrow at least $4 billion in 1986 from international financial institutions such as the International Monetary Fund and the World Bank, and from U.S. and other foreign commercial banks.

Both U.S and Mexican officials said the meeting in the Mexican city of Mexicali would focus on economic and other bilateral issues rather than on foreign policy. But the U.S. officials clearly wanted to remind Mexico of continuing concern in Washington over this country's neutralist, Third World orientation in international diplomatic forums.

Diplomatic observers noted that this U.S. approach to the summit could be interpreted in part as a way to take advantage of policy differences within the Mexican administration. The U.S. officials focused their criticisms on Mexican positions that are espoused most prominently by Foreign Secretary Bernardo Sepulveda, who is the most visible left-leaning member of de la Madrid's Cabinet.

The U.S. officials took pains to point out that some of Sepulveda's positions are not shared by the entire Mexican government. One U.S. official noted that Sepulveda has taken a harder line on the debt issue than Treasury Secretary Jesus Silva Herzog, who is a favorite in Washington.

The half-day meeting will be the third between the two presidents since de la Madrid took office on Dec. 1, 1982. It has been dubbed the "border summit" because Mexicali is just across the international frontier from Calexico, Calif.

Neither U.S. nor Mexican officials held out hope for major progress at the meetings. Earlier hopes have disappeared for announcement of a breakthrough on a dispute of several years over tuna fishing rights in border waters, U.S. officials said.

"The key is that they get together," the senior U.S. official said. "It gets people focusing on the issues."

The U.S. side will be looking for "a show of support from Mexico" on the so-called Baker plan for aiding heavily indebted Third World nations, the senior U.S. official said. The plan, proposed by Treasury Secretary James A. Baker III, calls for fresh loans of $29 billion during the next three years in return for economic adjustment measures on the part of borrowers.

So far, Mexico has welcomed parts of the Baker plan, such as its emphasis on economic growth rather than austerity measures. But Mexico also has joined other Latin American countries in criticizing the plan as offering too little new cash.

There were signs that the Mexican government might adopt a more aggressive line on the debt issue by pressing harder for relief from the burden of interest payments, U.S. officials said. So far, however, Mexico has not followed the examples of Peru and Brazil in running its economy independently of conventional IMF prescriptions.

The U.S. officials acknowledged that Mexico has made changes that have narrowed its differences with Washington over Central America during the past year. Mexico has cut back sharply its subsidized oil shipments to Nicaragua, for instance, and has restored full diplomatic relations with the U.S.-backed government of El Salvador.

But the senior U.S. official characterized these changes as relatively minor. He criticized Mexico for supporting resolutions sympathetic to Nicaragua in the United Nations, and of indulging in what he called "name-calling" there against South Africa and Israel.

"I don't see an overall pattern of a shift" in Mexican foreign policy, the official said. "What we want to see is some actions."

There were indications that some U.S. Embassy personnel and the Mexican government felt slighted by what they see as the White House's lack of interest in the meeting. Several embassy sources and Mexican officials suggested that Reagan should have scheduled more than four hours for the sessions.

"The White House just slipped this into Reagan's schedule because it was convenient," an embassy source said. The president will stop in Mexico while returning to Washington from his California vacation.

In addition, Mexico wants to give more visibility to the meeting than the United States does, U.S. and Mexican officials said. Mexico was pushing for a joint press conference by Sepulveda and U.S. Secretary of State George P. Shultz after the sessions, while the United States wanted just to issue prepared statements and then hold "background" briefings for journalists by officials whose names could not be published.