You can make an ersatz poured fondant with powdered sugar, yes. Some versions don’t even have to be “cooked”. However they’ll never be as good as a sugar syrup fondant because all powdered sugar contains cornstarch (about 3% by weight). The effect, in the case of an uncooked fondant, is to give the icing a vaguely starchy taste. Cooking gets rid of the taste, though it doesn’t get rid of the starch, which still makes the icing a bit chalky-feeling in the mouth.