Actually what you are seeing is a common problem with using quad meshes with the cloth room. This is an old problem. Despite the intent of the Poser Props 'High Resolution Plane' or cloth plane being used for cloth simulations it's actually not very good in the cloth room and in fact you will yield better results from Delaunay Triangulated or Tesselated meshes. What is happening is that the simulation is causing non planar faces to appear in the quads. Yes, subdivision can be used to help a bit with this with quad meshes. Generally if I create a quad mesh with the intent of using it in the cloth room I subdivide it an additional level beyond what would typically be required. 

The artifact you are seeing and various other cloth room quirks and experiments are examined at length in this thread...

Dynamic cloth - the cloth room For Compleat Dummies