Mortality is definitely the big theme in "I Heard a Fly Buzz When I Died," its whole reason for existing. Dickinson uses the poem to explore all kinds of things about death. She thinks about how it might feel, how it tends to happen, what we expect from it, etc. She looks at the idea from a bunch of different angles – before, during, and after the moment of death – and maybe tries to get us to think about it in new ways.
The speaker of the poem wants to talk about death, but not to explain it away. The cryptic language she uses to describe the moment of dying is meant to emphasize the mystery and the strangeness of the experience of death. Rather than reassuring the reader about death, this poem seeks to trouble and disturb.