The speaker writes to "Nobody," someone who feels as invisible and unwanted as they do. Both have been told they are ugly, and the speaker sees their own pain reflected in this other person. As the poem moves on, Nobody sinks into sadness, a grey life, and numbness. Then comes the loss: the speaker finds a note saying Nobody overdosed.
After that, the tone shifts from shared hurt to grief. The speaker remembers only a "sweet laugh" and wishes Nobody hadn't believed the cruel things they were told. The poem also turns into a kind of survival tool. The speaker writes so they won't take their own life, sending letters that go to "nobody" because the person is gone.
The mood is heavy, raw, and lonely, but there is a small thread of hope. The repeated "Dear Nobody" feels like a habit of reaching out, and writing becomes the way the speaker holds on. The poem deals with suicide and self-harm, so it may be hard to read.