Do petunias in a pot need to be cut back now there not looking as good as they were?

Yes - you need to pinch them back so that they grow bushy and not vine like. To do this trim back to the lower leaf areas. Not too far back though. And as soon as a blossom withers pinch it off just below the base of the flower to encourage a new one to grow. These are beautiful flowers but I don't plant them anymore because they demand more time than I have to give.