Antonio da Silveira, Brazil's foreign minister until last week and an outspoken critic of the Carter administration, has been appointed as this country's next ambassador to Washington.

The announcement was made yesterday within hours after Vice President Walter Mondale left here at the end of a two-day goodwill visit that was timed in part to demonstrate the administration's desire for closer ties with Brazil, which had been thought possible in the wake of Silveira's departure as foreign minister. Mondale returned to Washington today.

Both American and Brazilian diplomats, including Brazil's new foreign minister, Ramiro Saraiva Guerreiro, said the Mondale visit had contributed to improved relations between the two countries and that Silverira's appointment as ambassador to the United States should not be viewed as detrimental to better ties.

There was some informed speculation here, however, that Silveira may have sought the post as ambassador to Washington as a way to monitor whatever new relationship develops between the two countries over the next several years.

Silveira, 61, is known as an unconventional diplomat because of his acid tongue and his willingness to speak his mind in public. Last year, for example, he said shortly after a visit to Brasilia by President Carter that "you have to be a magician to understand the signals coming from Washington these days."

Silveira, who had a warm relationship with former secretary of state Henry Kissinger, made no secret of his view that the Carter administration has been indecisive and clumsy in its handling of foreign policy.

Two years ago, relations between the United States and Brazil sank to their lowest point in decades after the Carter administration criticized Brazil's human rights record and sought to stop it from purchasing sophisticated nuclear fuel reprocessing technology from West Germany.

Silveira, who was foreign minister at the time, said he thought the United States was interfering in Brazil's domestic affairs. As a result, Brazil canceled a 25-year-old military assistance program with the United States.

A high-level official in Washington reportedly said, when informed last month that Silveira might be appointed by Brazil's new military leaders as ambassador, "They wouldn't dare."

Silveira's appointment, however, has been formally accepted by the State Department.

Guerreiro said in an interview that Silveira's appointment "shows the importance we attach to our embassy in Washington." It is normal procedure here for a retiring foreign minister to serve, immediately after his tenure, as an ambassador.

The Brazilians consider it a particular mark of respect for the country to which a former foreign minister is sent.

As foreign minister, Silveira sought to develop Brazil's economic and political ties with Third World nations, particularly in the Middle East and Africa. He also sought to lessen Brazil's dependence on the United States as a trading partner, as a source of capital for industrial development and as a political and military ally.

In a speech last fall at the Woodrow Wilson Center in Washington, Silveira said that the United States historically had regarded Latin American nations "more as an object of policy than as a partner in open and frank dialogue. At least as fas as the Latin American countries are concerned, this approach is clearly exhausted."

While not all Latin nations have been able to decrease dependence on the United States, Brazil, brcause of its increasingly powerful economy and growing role in world trade, has been able to assert itself and develop independent relationships with other developed and developing countries around the world.

In just five years, Brazil has carved out an important place for itself in the export of nontraditional agriculture products such as soybeans and conventional arms. Meanwhile, European nations and Japan have replaced the United States as the largest source of foreign investment and capital to fuel Brazil's industrial growth. CAPTION: Picture, ANTONIO DA SILVEIRA . . . unconventional diplomat