Actually, the mesh density looks fine, you just need to cut in a few loops, and redirect some others. If you're going to animate this, you'd definitely need those extra edges around the mouth.
See my paint-over here:
Red: Create a loop
Green: Delete an edge
Blue: Suggestions for movement of the edges. You usually want your faces to be a square as possible, and have their sizes evenly distributed across the mesh.
Imagination is more important than knowledge.