The good/evil dichotomy doesn't appear to have much purchase in the pagan world of King Lear. Good and evil as we commonly understand them today derive largely from the Judeo-Christian tradition, so they are not really intrinsic to the action as it unfolds. All we can do, then, in examining King Lear, is to project our notions of what constitutes good and evil onto the characters and their actions.
If we say, then, that according to the moral standards of the Judeo-Christian tradition, characters such as Regan and Goneril are indeed evil, then it can be seen that they come to grief not through the superior forces of good but because of their own evil. Goneril murders Regan and then commits suicide—not out of remorse, but because she realizes that the game is up and that she's lost the battle. There's certainly no sign of good triumphing over evil here.
By the same token, one could argue that whereas Regan and Goneril are undoubtedly evil, Cordelia is the nearest thing in the play to someone who is unequivocally good. Even if we accept this assessment, however, Cordelia's goodness doesn't do anything to mitigate the dark forces of evil as represented by her sisters.
In the overall scheme of things, Cordelia's goodness is pretty much useless. If Regan and Goneril are ultimately defeated, it's not because they've been vanquished by the forces of good, but because they've been overpowered and outsmarted on the field of battle, by a military force that is as motivated by the desire for power and territorial acquisition as they are themselves.