Would the shaking of an earthquake be visible to somebody in a low-flying aircraft?
It depends largely on how deep, or shallow, the earthquake is, how large, and what kind of terrain you're looking down on. Some fairly large earthquakes, magnitude 6-7, can have very little surface effect, when they are centred 5+km down as many earthquakes in New Zealand often are. The finer the sediment on the surface, and the thicker the sediment cover, the greater the ground shaking is at the surface and the more motion is usually visible. Different ground covers will also make shaking more or less visible, long grass, or cereal crops, on a windy day will look like they're in motion even without a 'quake.

Shallow earthquakes often result in apparent L-wave activity, by which I mean that you can actually watch the ground wave coming towards you if you're outside on relatively flat ground, even during relatively small 3-4 pointer 'quakes. These waves also make parked vehicles rock from side to side, I would expect these effects to be the most visible from the air in the case of small to moderate earthquakes.

Larger 'quakes will cause noticeable shaking of buildings, power poles, aerials etc... even when quite deep and possibly distortion to large linear features such as roads, railways, waterways, and crop and orchard rows. These distortions would be visible as they form, and, when large enough, from very high in the air.