Yoghurt would be better used in place of buttermilk, cream or other wet ingredients, rather than to sweeten it. As someone else said, it would make the recipe very wet - obviously it would depend what other ingredients were in there, but to replace the sugar in a recipe, you'd need to add something else to compensate, and that would usually be flour to thicken.
Good luck experimenting though and let us know if you find any great recipes!