"Unselfie" plays with the idea that a photo of yourself can never be all of you. The speaker begins with a worry: if I am only my picture, then I am nothing more than that flat image, and there is nothing left behind it. The rest of the poem pushes against that fear.
The speaker says no face is ever finished, so a self can't be fixed in a single frame. The "fat and home" face seems to suggest someone comfortable, even a little worn, who has settled into the "general tow of people," a phrase that pictures a crowd as a current pulling everyone along. Being part of that pull could feel like disappearing, but the last line turns it around: "and yet part sea." The speaker is one drop in something large and alive, not erased by it.
The mood is quiet, thoughtful, and gently relieved. It starts with unease but ends by finding a kind of freedom in not being a fixed image.