It is not a nonsense poem because it doesn't employ rhythm and rhyme. A Nonsense poem is humorous and with absurd characters the poem does not have meaningless made up words as a nonsense poem would.
The message of the poem is about the boy who would never do as he is told or what is expected of him. He always left the door open despite all that has been said and done.
This poem is basically about our human nature. People delay and do not give attention to specific things or details till a form of punishment or penance is given to them, then they finally acknowledge the work and start doing it.
