There is a hard rock chart, and that includes metal releases. But this is an aside, although there are plenty of metal albums that have charted fairly high in the overall Billboard 200 (Pantera's Far Beyond Driven at #1 certainly comes to mind).
Mastodon is certainly pop culture, based on the the fact that they have albums chart fairly high (#10 on Billboard 200 for their latest), and have sold hundreds of thousands of albums.