In a major effort to improve relations between the United States and Brazil, Vice President Walter Mondale today backed away from a confrontation over this country's nuclear energy program, which includes plans to build a uranium reprocessing plant capable or producing weapons-grade plutonium.

The vice president, here on a goodwill visit that included meetings with top Brazilian opposition and government leaders, including President Joao Baptista de Figueiredo, said that the two-year controversy over the Carter administration's effort to stop West Germany from selling nuclear reprocessing technology to Brazil had been "greatly exaggerated."

Mondale also praised what he said was a significant improvement in the human rights situation here and Figueiredo's promise to return the country to democracy when he leaves office six years from now.

Mondale did not mention the nuclear issue in a prepared statement, stressing instead the "great importance" the United States attaches to its "constructive, mutually beneficial relationship" with Brazil, which has the world's 10th largest economy and which Mondale said is now "a major factor in regional and world" affairs.

When asked whether U.S. opposition to the nuclear reprocessing plant had come up during his talks with Figueiredo, Mondale said it had and added that the Carter administration's policy "has not changed." At the same time, Mondale said, Brazil "has made it clear that she has no interest in developing nuclear weapons."

Both Brazilian and U.S. diplomats here stressed even before Mondale arrived Wednesday night that neither government had any interest in reopening the nuclear controversy, which, along with human rights, resulted in a major deterioration in U.S.-Brazilian relations after President Carter assumed office in 1977.

At that time, Brazil canceled a 25-year-old military aid agreement with the United States to show its displeasure with what it considered interference in its internal affairs. There had been speculation that Mondale and Figueiredo, who accepted an invitation to visit the United States later this year, would discuss the possibility of renewing military ties.

But that subject was not discussed. Mondale said his meeting with Figueiredo centered on global rather than bilateral issues. The vice president said he briefed the new Brazilian president on the new Israeli-Egyptian peace treaty, the status of SALT talks with the Soviet Union and efforts by the Carter administration to reduce inflation.

Mondale said he was confident, as a result of his talks here, that Brazilian-U.S. relations would improve significantly in the years ahead, a sentiment echoed by Figuerido who was described as "very satisfied" with his meeting with Mondale.

The vice president went so far as to say during a press conference that there were now "no serious bilateral issues" standing in the way of excellent relations between the two countries.

Although Mondale denied the Carter administration has become more interested in Brazil as a result of the fall of the shah of Iran, American diplomats said that countries such as Brazil, as well as oil-producing Latin American nations such as Venezuela and Mexico, are viewed as increasingly important by the United States, especially in light of American reverses elsewhere in the world.

"Washington has finally awakened to the fact that Brazil isn't just the land of Carmen Miranda and 'On The Way to Rio,'" said one American diplomat.