The poem is strange and a little unsettling, and it's hard to pin down on a first read. A king holds "us" in his arms, lying down on a chair, and then he "breaks open" and hands over everything he has: his sons, his countries, his family, his ceremonies, even his neck and left leg. The king seems to be giving himself away piece by piece, as if his whole body and realm were something to be served up.
The last image is the sharpest. He is "like a ham made to do / in lieu of lobster." A ham is a humble stand-in for something grander, so the king becomes a cheap substitute, which fits the title. The poem suggests that power and royal dignity are not as impressive as they look, and that the king is making do with a lesser meal.
The mood is odd and darkly comic. The speaker seems to receive these gifts with a mix of bemusement and quiet irony, treating the grand and the ridiculous as the same thing.