The word "all" is a generalization, meaning "most." Alternate translation: "a laughingstock to most of my people" or "someone whom all my people mock" (See: [[rc://en/ta/man/jit/figs-hyperbole]])
Jeremiah speaks of being the person about whom they sing their taunt song as if he were the song itself. Alternate translation: "and I am the one about whom they sing taunting songs" or "and they sing taunting songs about me" (See: [[rc://en/ta/man/jit/figs-metonymy]])