Is it ok to use sour cream as substitute of sour yogurt?
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.