Music-minded Ethan Mordden (Best Foot Forward, Opera in the Twentieth Century) has rescored the Twenties as a historical melodrama and cast as its villain: jazz. ""Jazz was no mere overlay,"" he writes, ""in its feverish nihilism, its libidinous money-take, its rotten, egoistic energy, jazz essentialized the falseness of the new electronic America, the irresponsibility of freedom without ambition."" So some said at the time, and except for an overlay of post-Sixties venom, this could as well have been written around 1930: the themes and content--from Harding and Mencken to Al Capone and Peaches Browning--follow the interpretive pattern of the period. What sets Mordden's effort apart from Frederick Lewis Allen's far more equable Only Yesterday (1931) and J.C. Furnas' fuller Good Times (1974) is, indeed, its slant. Apropos of the repressive Palmer Raids, he comments, ""an efficient roundup of subversives was clearly called for,"" while Sacco and Vanzetti are as seen ""Communist puppets"" who captured ""the hearts of highbrows."" There's a certain novelty value to his nay-saying--where else today would you read: ""In truth, American participation in the League of Nations would have been a dangerously entangling venture""? And his Lindbergh chapter has more pizazz than most, for to him Lindbergh is still the hero of the day, its only hero: he had ""the confidence of the winner."" Its obverse, the object of Mordden's greatest scorn, is ""the fellowship of the underdog."" But enough: this is, as he says, ""idiosyncratic,"" for those who prefer to look back with displeasure.