The poem looks at a corpse with a cold, wry eye. Its opening claim is almost a joke: a dead body is perfectly well-behaved, since it commits no "breach of civility." The dead can't interrupt, complain, or say the wrong thing. The title, "Polished," seems to play on that idea of social smoothness, though it also hints at something finished and fixed.
Then the tone darkens. The mouth is "all drawn in," a detail that suggests the shrunken, tightened look of a face after death. The last lines make a grim comparison: the mouth is like "what teemed / at so late a supper." The word "teemed" brings in crawling, swarming life, and "so late a supper" suggests the body itself may become the meal. Judging by the imagery, the speaker doesn't seem horrified so much as detached and darkly amused. The mood is quiet, dry, and unsettling, with politeness and decay sitting side by side.