Indeed, a round peg objectively fits into a round hole, whereas a cube doesn't pass through the hole. In just that sense there would be objective fitness were an asteroid to smash into our planet and to destroy all known life. The fitness there would be a matter of decisive causality. The asteroid would cause earthquakes and conflagrations, which in turn would kill living things.

If we look closer at the cube and the round hole, and we leave aside any subjective preference for the pleasing effects, we'd have to say that the cube would fit the round hole in its way. The cube would clang off the surface of the hole, its corners "fitting" the hole by generating that sound.

Only if we presuppose the superiority of those effects that please us can we dismiss the asteroid's extinguishing of life or the cube's clanging off of the round hole as unfit or disharmonious.

If you're talking about fitness as adaptation or as a tailoring of means to ends, you'd be presupposing the appropriateness of a certain end, which in turn would be subjective. You'd be presupposing that life ought to continue, so that the asteroid impact would be unsuitable to life's continuation, which of course would be true. But if we assume that life ought to be extinguished, the asteroid impact would do the trick, so it would be adapted to that destructive purpose.