President Reagan's national security affairs adviser, Robert C. McFarlane, today brushed aside reports that Mexico had balked at endorsing Reagan's proposal for peace talks between Nicaragua and antigovernment rebels.

Asked about news accounts from Mexico City that indicated Mexico is withholding support for the Reagan initiative, McFarlane said:

"Well, I think when you read what they have said, they have said that they see this as perhaps making a positive contribution to the process. Well, I don't view that as a negative stance."

Mexico is one of four members of the Contadora group of nations seeking a negotiated settlement in Central America. Reagan spoke by telephone this week with Mexican President Miguel de la Madrid before Mexico issued a communique indicating that it did not want to interfere in Nicaragua's internal affairs.

Reagan has called for peace talks between the Sandinista government and the opposing "contras," linking such talks with his request for $14 million from Congress to aid the insurgents.

McFarlane said Panama and Venezuela have told the United States through "diplomatic channels" that "they think this is a very constructive step." Colombia, the fourth Contadora member, has endorsed Reagan's plan. The president has talked to leaders of all four nations in the last few days, he said.

McFarlane acknowledged that Mexico's response was "less effusive" than the others.

The security adviser said he thinks that a "majority" of the Nicaraguan people now oppose the Sandinistas and support the contras. He said the Sandinistas must deal with their "domestic opposition," not with the United States.

There are no plans for resuming direct U.S.-Nicaragua talks, he said. "The problem is between Nicaragua and its neighbors. The problem is also between the Sandinista government and its own people. It is not a problem with the United States."