Not really. It depends on which level of play you're at and how skilled you are. There's more traffic in GT5P (16 cars versus 6 in earlier GTs), but the Ai doesn't drive quite as stoopid as it did in earlier GT games. It seems more respectful and aware of your presence. For instance, as i'm trying to brake and enter a tight corner, I have yet to be truely "pitted" (basically spun) by a sim car.
But if you're not skilled at figuring out ways to dodge traffic, and there's 8 cars starting in front of you, yea, it will seem like a harder game.