POP ROCK The Moody Blues: ''The Present.'' (Threshold/PolyGram Records TRL-1-2902) - The cover of the latest Moody Blues album shows what looks like a couple of nymphs under a classical Greek column with a background of golden mountains - an obvious sign that the group hasn't abandoned its somewhat idealistic, mystical brand of rock. There have been subtle adjustments over the years that have helped to trim down what would today have seemed like excess weight. By now, ''Nights in White Satin'' sounds embarrassingly melodramatic (in an era of the abbreviated noises made by new-wave bands.) But gorgeous, just the same. Their style, for better of worse, is largely unchanged, however, since their glory days of the late 1960s. Why change a winning recipe? And there's a certain creditability, in my book, to a group that doesn't change its style to appeal to fashion. Yet, nothing on this effort seems to justify the drama implied by the instrumentals. The songwriting here is lackluster. Some numbers seem like colorless rewrites of past successes. And the starry-eyed lyrics, while universally warm and humane, have the solidity of angel food cake. And without well-constructed songs, the baroque treatment the Moody Blues gives its music just sounds moody. ''Blue World,'' one of the songs groomed for singledom, is not too bad, really. It has an encouraging message. Yet, like so many of the other songs here, it doesn't crystallize into anything worth this album's elaborate trappings.