The poem is a single, wry thought: if everyone in the world is a playing card, then some of us must be kings of hearts. It starts from a familiar comparison, life as a game of chance, and then takes a small sideways step into a joke.
The speaker sounds relaxed and a little amused. The words "I suppose" make the idea feel offhand, as if it just occurred to them. There's no big complaint about fate here. Instead, the speaker notices that a deck has ranks and suits, so if people are cards, some will end up with the grand, romantic-sounding roles. The king of hearts suggests status, but also love and feeling, which gives the line a gentle double meaning.
The mood is light, playful, and slightly ironic. The poem hints at luck and unequal positions without lecturing. It leaves us to wonder who the kings are, and who gets stuck as the lower cards, and whether the speaker counts themselves among the lucky ones.