The scene is a birthday party gone sideways. The "guest of honor" of the title is a woman who has had so much rum that she has passed out face-first in her own cake. The poem's quiet punch comes from the second line: no one pays any attention. The party carries on around her.
The tone is dry and deadpan. The speaker doesn't scold or cheer; they simply report what they see, which makes the image funnier and sadder at once. A cake is meant to be the centre of a celebration, and here it has become a bed. The word "honor" turns bitter, since the person being celebrated is ignored.
The last line, "birthday rum," works like a punchline and a shrug together. It names the cause without comment. What's left is a mood of awkward comedy mixed with loneliness, and a hint that the crowd may care more about the party than about the person it's for.