Can fermented products (yoghurt, creme fraiche etc) be added to a ravioli filling?
There is no reason not to do it in the sense of the food police arresting you or your oven exploding. But still, many people prefer not to do it (and some of them may feel that this gives them the moral obligation to become said food police :) ) 

The scientific reasoning behind it is that when the dairy proteins, when heated in the presence of the acid produced by the fermentation, curdle. The clumping is reduced if they are mixed with something else - e.g. you are using a dairy product that's high in fat, like creme fraiche, or your filling is not made mostly out of fermented dairy. But if you just dump yogurt into your ravioli, it will curdle during the cooking, basically turning into a kind of tvorog. The filling will be grainy and swim with whey. 

If you are bothered by that change, don't do it. If you are not, you might still find your ravioli easier to handle if you fill them with tvorog outright instead of using a filling predominantly made of fermented dairy. There will be small differences in the result however, and if you find out that you like it, then there is no reason not to do it. 

If you have a filling mostly made of something else, and want to add small amounts of fermented dairy, you probably won't notice any unusual effects happening anyway.