I also think it would be pretty difficult to substitute artificial sweetener for powdered sugar in frosting. If a frosting recipe calls for granulated sugar, if you use one of the sweeteners that are used cup-for-cup, those contain fillers and the ones I have seen are pretty high carb.
Here is a link to the cream cheese frosting recipe I use. It calls for 8 pkts. Splenda, which is fine for me, but you can sweeten it to your taste. I often substitute chocolate, strawberry or maple sugar free syrup for the heavy cream, and sometimes add coconut and/or chopped nuts, so it is a very versatile recipe, if you want to give it a try. But pretty much any low carb cream cheese frosting recipe is good.