In most things, No. Powdered sugar has cornstarch in it and has much less sweetening for the same amount.Clarification:Powdered (confectioners) sugar has only a minimal amount of cornstarch, which...
It's not a good idea. Confectioner's sugar is powdery and regular sugar is grainy. If you are topping something with confectioner's sugar it would be a good idea to go buy some.