In just a few lines, the speaker sets two ideas of themselves against each other. First comes the body, the "carcass," which is "contrary as lead": heavy, stubborn, dull, and doomed to "amount to nothing." There's a blunt, almost shrugging honesty in that line. The speaker doesn't dress up mortality, and the word "carcass" makes the body sound like something already dead.
Then the poem turns on "even though." Whatever the speaker is beyond the body, it feels "as clear as / the clearest wind." That image is light, invisible, and free, the opposite of lead. The poem seems to hold both truths at once: the physical self is heavy and temporary, while the inner self feels open and transparent.
The mood is quiet and a little wry. It isn't despairing, but it isn't triumphant either. The title, "Mere mortal," adds a note of humble self-mockery, as if the speaker accepts being small while still sensing something airy and unburdened inside.