L.A. winners: on the diamond. . .

The cheers greeting the 1981 World Series champion Los Angeles Dodgers are fitting tributes to a team that proved that the underdog not only can come from behind and win - but win big. In New York, meanwhile, the stunned disbelief that marked the Yankees' collapse from a two-game lead to a shattering four-game wipeout is heard in almost plaintive appeals of ''say it isn't so, George!''

Yet, neither George Steinbrenner and his (usually) feisty New York Yankees, nor baseball fans in general, need feel ashamed in the outcome. With its unexpected result it was perhaps an appropriate climax for an incredible summer marked by the first in-season strike ever, a near-league-victory by Mon-treal, which would have produced the first-ever international series, and a year in which, as one close fan put it, ''long-term baseball strategy was short-circuited in the first season, and didn't apply in the second season. ''

But after all the statistics were totaled up was there not something reassuring about the very fact that the Dodgers and the Yankees were squaring off against each other for the hearts of fans, as they have so often over the years, ever since the glory days back when the two teams vied for the affections of their fellow Gothamites and the Dodgers wore Brooklyn uniforms? Here, after all, was the continuity of baseball, marking yet another summer. Some perceptive social historians argue that to understand the United States you have to understand baseball. Coming on a day that also saw President Reagan win an unexpected big one on the AWACS vote, the Dodgers' final sock-it-to-them dazzling victory over the Yankees suggests the social historians are measuring the bases pretty well themselves.

The 1981 season may go down as a footnote. But it deserves bold type.