Prime Minister Robert Mugabe said today he was optimistic that President Reagan would be more effective than former president Jimmy Carter is pressing for political change in southern Africa and in moderate tones separated himself from the administration's sharpest African critics.

The prominent southern African leader said in an interview that he nevertheless was worried about the lack of progress toward independence for South African-controlled Namibia, and believes action should be taken to arrange a peaceful solution within six months. He also criticized the United States for allowing a controversial tour in September by a South African rugby team, and differed with President Reagan over the pace of change in white-minority-ruled South Africa.

Unlike many other African leaders, however, Mugabe expressed a remarkably conciliatory outlook toward the new U.S. government, and appeared to be willing to give the administration more time to prove itself in the area, although he took care not to praise actions taken to date by the Reagan team.

Mugabe's continuing moderation is important because the United States has signaled him out as ts "point man" in dealing with southern Africa. He was the only black African leader met by Deputy Secretary of State William Clark on a trip to the continent in June, and U.S. aid to Zimbabwe under Reagan has been sharply increased.

The general tenor of Mugabe's comments today is bound to be welcomed by U.S. officials, who came under sharp fire at June's summit meeting of the Organization of African Unity. The OAU said the United States had entered into an "unholy alliance" with South Africa and blamed Washington for the stalemate on Namibia. The statement provoked a heated riposte from the State Department.

In soft, measured phrases, Mugabe said about Namibia, Africa's last major decolonization problem: "Nothing is happening, but this is not to say nothing will happen."

"One does not lose hope," Mugabe said. "One would want to see something done in the next six months or so."

Although "happy" about bilateral relations with Washington, "what we find lacking is an enunciation of U.S. policy on southern Africa," he said. U.S. officials, he said, "hae come to the stage where they must translate their experience into definite policies . . . and say what their policy is."

Mugabe said Reagan was in a better position than Carter to exert pressure on Pretoria because the South Africans had "set their store" on him because of his favorable remarks during the election campaign.

He was hopeful that U.S.support for Zimbabwe and continuation of some of Carter's policies in the area were indications that those earlier pro-South Africa views "will stand to be changed and be realigned with the general opinion of Africa."

"I believe Reagan is a principled man," Mugabe said, adding that if the president held to traditional American democratic ideals, "there is no reason why he cannot stand firm" and persuade South Africa "to act in accordance with the international community" by granting Namibia independence.

Little such optimism has been forthcoming elsewhere in Africa or from African interest groups in the United States, where it is charged that the Reagan administration has made considerable concessions to South Africa without gaining anything in return.

"I don't believe the United States has done much yet" that either favors or pressures South Africa, Mugabe said, despite visits by South African officials to Washington, the rugby tour approval and indicationsof future expanded cooperation.

He added, however, that it was "incongruous" for the United States to try to develop good relations with both South Africa and black Africa. "I don't think the two go together. It's incompatible," he said.

Asked about Reagan's view that South Africa was seeking to bring about changes for blacks, Mugabe said bluntly, "I don't see even cosmetic changes taking place."

"We do hope the United States will not act in a negative way on South Africa" by supporting apartheid, he said. "That is the only area which could ruin our own relations." He added that any U.S. effort to change the U.N. plan to bring about Namibian independence would raise opposition throughout Africa.

Mugabe called the American decision to admit the South African rugby team an "unfortunate stand" which is "in defiance of" Africa's efforts to prevent sporting ties with Pretoria because of its policy of racial separation known as apartheid.

"One does not expect a country like the United States, which enjoys full democracy, would allow itself to be used as an arena for the exhibition of apartheid. I find it difficult to accept," the prime minister said.

He added, however, that there is still time for Washington to review its decision and declined to say whether he would favor a possible African boycott of the 1984 Olympic Games scheduled for Los Angeles.

His own country's relations with Pretoria were deteriorating, he said, because "South Africa is apparently trying to use our economic relations to further political purposes, which is the very thing South Africa does not want other countries to do vis-a-vis it."

Eighty percent of Zimbabwe's trade goes through South Africa because of commercial routes developed during white rule. Mugabe said he was "suspicious" that South Africa was causing delays in vital supplies to pressure Zimbabwe politically.

On domestic mattes, he sharply criticized white former prime minister Ian Smith for "talking quite racially" during recent white by-election campaigns.

"He should not behave as if he were a hero. He isn't. He is a defeated man," Mugabe said.

His meetings with Smith, he said, have been forums for the former prime minister "to express what he regarded as the fears and apprehensions of the white community," which makes up less than 200,000 of the 7 million population.

"What about the fears and apprehensions of the African community?" the prime minister added.

Mugabe also attacked the white business community for allegedly trying to maintain a supply of cheap labor. Noting the failure of companies to grant significant raises to workers above the recently imposed minimum wage, he said. "This is how they cheat." The government, he added, "must abolish this concept of cheap labor."