Rich Mansfield's speaker is putting on a show. They smile and laugh for everyone, for "the world," for "you," even for themselves, but none of it is real. The repeated "it isn't me" sounds like someone trying to separate the face they show from the person underneath.
The second half explains why. The speaker says there is "no happiness in my heart to stoke," and compares that heart to a fire in a hearth. Fires need tending, and this one has gone out: "the flames have died." The simple, chanting rhymes (hearth/earth, died/inside) make the poem feel almost like a sad nursery rhyme, which suits its flat, tired tone.
The mood is hollow and weary. There's no shouting or drama, just the quiet exhaustion of faking cheerfulness when nothing inside responds. The title's word "useless" fits: these smiles do nothing for the speaker, and perhaps nothing for anyone else either.