The beauty of haikus is that they leave so much open to interpretation. I am not exactly sure if what you had in mind was similar but here's the way I looked at it. It is a startling depiction of how unconscious the smoker is that she is killing herself with every little puff. She is creating her own grave, around which the pigeons are pecking at the ash. The ash they peck at is bits of the smoker herself that have died because smoking kills slowly. In that sense, pigeons here are really vultures, but the smoker sees them as benign pigeons.