Mexico City — Critical reaction is emerging here to a new economic message from the Reagan administration to the countries of Africa, Asia, and Latin America. The message is being spread by tall, prim James Buckley, the present head of Radio Free Europe and Radio Liberty and a former Republican senator from New York, as part of a shift in United States family-planning strategy at the 140 -nation United Nations population conference.
Mr. Buckley, head of the US delegation, says that the third world needs fewer government controls and more free-market incentives and reforms. His contends that:
* More people in the world don't necessarily mean less growth.
* It is nations such as South Korea which have ''the freest market economies'' that show the greatest ability to ''absorb growing populations and provide higher standards of living....''
* Men and women should be freed from government controls, food price fixing, and high taxation; and ''personal industry and thrift'' should be encouraged to enable economies to ''adjust to changes in supply and demand.''
* The Reagan administration ''does not believe that we face a global population crisis.''
In Monitor interviews, third-world participants were skeptical. They said that their countries' slums, villages, and fields spelled out a different message.
''In Africa we have countries where only 8 percent of adults are literate,'' said the leader of an influential African delegation who asked that his name be withheld.
''Who else is there except the government to organize industry and education and health care? It's simplistic to say that the government should get off people's backs.
''If Africans are left alone and not helped, their populations will keep growing rapidly and their economies just won't be able to grow....''
An Indian participant commented: ''Where you have 50 percent of the people in countries like mine below the poverty line, how can you say the answer lies in getting government out of the picture? It is government that runs the programs that support poor people, and which must be involved in economic planning as well.''
A number of Asian delegates took for granted that personal initiative was extremely important.
But, they said, unless population growth rates fell, millions of people would be so poorly educated, housed, and fed that they would be unable to fulfill their potentials.
''To many of us,'' added an official from southern India, ''the Buckley free-enterprise message is in fact a call for US multinational corporations to come into our countries.
''We don't want that. It would be good for the US perhaps, but how would it help our people?''
Another Asian added: ''This US economic message is doctrinaire. It doesn't agree with the facts of the way we live.''
Delegates were not impressed with Mr. Buckley's use of South Korea as an example of free-market growth.
''What he doesn't say,'' said the African delegation leader, ''is that the South Korean economy is highly Americanized, and that the country has a successful family-planning program which has slowed down population growth and allowed economic gains to take effect.''
Asked by reporters whether he agreed that there was ''no population crisis in the world,'' the executive director of the UN Fund for Population Activities (UNFPA), Raphael M. Salas, replied: ''The interpretation of 60 percent of third-world countries, containing 80 percent of third-world populations, is that something has to be done about stopping growth in population rates.''
Family-planning advocates from the US also took strong exception to White House economic reasoning.
''Anyone who has been to Bangla-desh,'' said Fred O. Pinkham, president of the Population Crisis Committee of Washington, ''knows that almost 99 million people live there in a country the size of Wisconsin.
''By the year 2000, the population will be 145 million, and by 2025, an incredible 219 million.''
People with high education and incomes always had more access to contraceptive information and methods, he said. The same access now needs to be given to poorer peoples. ''There is a great unmet demand for smaller families,'' Mr. Pinkham added.
Werner Fornos, president of the Population Institute of Washington, saw the Buckley language on economics as ''pure rhetoric - unless the administration backs up its words by lowering tariff barriers against third-world exports and sets up a free-enterprise support fund of at least $1 billion to be loaned out at 4 percent interest.''
Just before Mr. Buckley's formal address to the Mexico City conference, the Population Institute planned to try to counter his words by presenting to the UNFPA's Raphael Salas a statement signed by a number of third-world leaders.
The statement called for new efforts to stabilize world population growth as soon as possible.
Meanwhile, Washington anti-abortion lobbyists supported the other part of the Buckley message: tightening of control on US family-planning aid to private groups that use any money at all - US or not US - to support or promote abortion.
But family-planning advocates feel they have won a victory because Buckley also reiterates that the Reagan administration will continue spending all family-planning funds appropriated by Congress.