Yes, like Claire mentioned, it is a matter of the Resolution of the Mesh in the lips area. The Simplify Brush can help there.
But it might also be a good idea to start over. It depends on how much Sculpting experience you have. I myself re-started the first Chapter of this Course 5 times, or so and each time it got easier and the result looked better.
And like Claire, I also tend to make Incremental Saves per Chapter, or per important milestone.
