Using AutoMapper in ASP.Net MVC
If you were not using AutoMapper, your POST method would either have a block of manual mapping, or it would call out to a custom mapper class. In the latter case, you would have a line (or two) calling that helper class instead of a line calling AutoMapper.

Whether returning a PartialView is the best method depends on your UI; if that's what your View needs, then that's what you need to to. (I personally don't use them in my markup.)

I wager the reason you think the mapping doesn't belong in your POST method is because your Controller is not just concerned with Control, but also Persistence and Construction. If the rest of your application is indeed this simple, then you might as well stick with it, but as complexity grows, you should consider hiding the persistence concerns behind an interface.