Is the North American mastodon an autotroph?

No: an autotroph is an organism that gets its energy without consuming other organisms (auto = self, troph = feed), whether plants or animals. In order to grow, autotrophs need a "food" source, just like the rest of us. They most commonly use sunlight (plants), and sometimes chemical reactions (chemosynthetic bacteria, for example). A mastodon eats plants, plants are organisms, so a mastodon is an herbivore, but not an autotroph.