An old general brought out of retirement to be the commander in chief during Napoleon's invasion, Kutuzov gets some of his reputation restored as another one of Tolstoy's historically real characters.
While Tolstoy brought Napoleon down a notch in this novel, with Kutuzov we see that the bringing-a-mythological-man-down-to-earth thing can actually work both ways. In the historical understanding of the time, Kutuzov was a short-sighted and cowardly man who somehow got himself appointed to lead the army, and then almost lost to Napoleon before weather and luck got the better of the French. But by putting the man into War and Peace as a character, Tolstoy is able to turn Kutuzov into a wise old soul who can commune in some mystical way with the general day-to-day feel of the army and guide it along the right path.
What's interesting is that Tolstoy rehabs Kutuzov's reputation without trying to whitewash his flaws. We still see how old, out-of-shape, tired, and even lazy the man is. Tolstoy's main point is basically that, just as Napoleon wasn't some larger-than-life superhero, so Kutuzov wasn't just a blundering imbecile. By showing us the human characteristics and the multi-dimensionality of these and other historical figures, Tolstoy takes the Manichean tendency out of history. (Manichaeism, by the way, refers to any belief system that has a black-and-white, good-and-bad approach to morality, skipping over any nuance or gray areas.)