The speaker steps outside, or maybe just opens a window, and gets hit with two unpleasant surprises at once: cold sleet and the smell of cow manure carried on the wind. The opening "Hey" sounds like a half-laugh, as if the weather has caught them off guard and they can't help commenting on it.
The last line changes everything. "Goodnight, moon" echoes the bedtime ritual of saying goodnight to the things around us, and here it's said to a moon that is presumably hidden behind the sleet clouds. Judging by the tone, the speaker isn't bitter about this. They greet the muddy, smelly, rough evening with a kind of fond humor and accept it as it is. The mood is wry, down-to-earth, and oddly tender. The poem suggests that a night doesn't need to be pretty to deserve a goodnight, and that the countryside, smells and all, can still feel like home.