The topology of the human face is quite problematic, and the corners of the mouth are especially tricky to get right. I analysed the construction of the mouth and the behaviour of its skin to arrive at this solution. It is efficient and effective.
Use this model as a reference or merge it with an existing poly face for a quick fix!
The uploaded scene includes the model as shown in the accompanying preview. That is, the basic poly mesh is there, converted to SubD, textured with procedurals, simply lit, bound to a stand-in skeleton, tweaked with clusters, and animated in an open-close loop. Any or all of these features are easily deleted to leave the base mesh itself, the topology and construction of which is the main item of value. Note that the poly and vert counts refer to this poly base, not to the tesselated SubD version.