can I substitute greek yogurt for sour cream for the topping on a baked cheesecake?
It's difficult to say without considering the details of a specific recipe.
Both items have a sour dairy flavor. Obviously the two products often also taste different, because they may use different bacterial strains and temperatures in fermentation. But if you are using them as an ingredient in a topping that has other strong flavors, the taste difference may not be that noticeable.
In general, the main difference is going to be the fat content. Full-fat greek yogurt typically has about twice as much fat as regular milk, around 10%. Sour cream generally has around 18-20% milkfat (though some varieties may be higher).
Aside from less richness in your topping, the lower fat content of the greek yogurt could impact the texture, perhaps making it more runny. On the other hand, the thickness of greek yogurt is due to a higher protein content (rather than fat) which could also impact texture. You might need to make adjustments in quantity and/or add thickeners or liquid to obtain a similar texture to what you normally expect.