The speaker feels reduced to a joke by the people around them. Nobody believes they have real emotions, feelings, or thoughts, so they become something to pull out when a conversation goes quiet or when someone wants a laugh. The poem is blunt about how much this hurts: the speaker says they aren't taken seriously, aren't seen as human, and are labeled "different" and "a failure."
The repetition of "joke" does most of the work. The word keeps coming back, almost like the speaker hears it everywhere, and the short, plain lines make it feel flat and worn down rather than angry. The last lines add a bitter twist: of all the labels people use, "joke" is their favorite. The trailing dots at the end leave the thought hanging, as if the speaker is too tired to say more. The mood is lonely and quietly heartbreaking, a portrait of someone who wants to be seen as a person and isn't.