The Greatest Slump of All Time, by David Carkeet. New York: Harper & Row. 232 pp. $14.95. THE underlying idea of David Carkeet's second novel is very clever. All the players on one of baseball's best teams are deeply depressed, wallowing in despair and talking about same constantly. At first it's all very funny - there are some laugh-out-loud scenes - but it goes on a little long, and Carkeet's novel is also handicapped by his having to juggle so many characters. ''The Greatest Slump of All Time'' is an eccentric and good novel with a mood that suggests a combination of the Woody Allen of ''Annie Hall'' and the Bernard Malamud of ''The Natural,'' but it doesn't sustain its premise quite well enough to be more.