D.C. Mayor Marion Barry said yesterday that regardless of whether Walter Mondale wins the presidency, the record number of blacks registering to vote as Democrats this year will have a major impact on the outcome of future local, state and congressional elections leading up to the 1988 presidential race.

Appearing on a panel with other black mayors at a Howard University student forum, Barry credited the Rev. Jesse Jackson's presidential bid during the Democratic primaries with inspiring blacks to get more involved in the election process.

At the same time, he disagreed with those who suggested that President Reagan is so far ahead of Mondale in the polls that the black vote this year won't make a difference in the Nov. 6 election.

"I think the black vote can make a difference and will make a difference," said Barry. "I challenge the assumption that Walter Mondale is 12 points behind."

Barry, Hartford Mayor Thirman Milner and six other black mayors from smaller towns or cities participated in the Howard forum in conjunction with the National Conference of Black Mayors, whose board members were in town for a meeting.

The mayors' conference is a nonpartisan service organization organized to provide technical and management assistance to the nation's 268 black mayors. But the mayors who joined the panel, including Barry, the group's president, all said they are supporting Mondale .

While conceding that Jesse Jackson's political agenda, particularly his call for economic sanctions against South Africa for apartheid, has been largely ignored so far by Democratic policy-makers, the mayors took some comfort in the fact that Jackson has at least raised issues that are important to blacks.

"You don't always succeed in getting everything you want on the front burner," said Barry, who expressed disappointment at Mondale's "lack of aggression" on the apartheid issue in his foreign policy debate with Reagan.

At the board's final meeting for the year, Barry announced that the group will send a delegation of 15 black mayors to China next month for business and tourism talks. Barry, who has already visited China, will not be among the delegation.