Sour cream could work. It has a similar flavor to plain greek yogurt, although the consistency is somewhat different. In fact, My fiance and I have switched to using greek yogurt in place of sour cream because of this since sour cream is higher in calories. I think it would be just fine for a sauce.
Depending on the specific application, you may (and probably will) get good results, but the flavor and texture will be may be slightly different when you substitute sour cream for unsweetened, unflavored yogurt.
Consider that both of these products are fermented dairy. The main differences are going to be the level of fat (based on the specific dairy item from which the product was fermented), and the level of acid.
In many applications, because the characteristics are very similar, the outcomes will be very similar.
In some uncooked sauces like tzatziki, you will probably get a delicious result, but it will be different since sour cream tastes slightly different.
In cooked sauces like stroghanoff or some curries, again, the result will be similar but different. The one specific issue you may face is whether the sauce breaks, which will be dependent on the level of dairy fat in the sour cream or yogurt, not which one is being used. So you may wish to use a full-fat sour cream for cooked sauces.
The one place where they are nearly freely substitute is baking, especially quick breads, where they perform nearly identically. The flavor differences are not dominant.
Best Answer
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.