1. It is nonsense poetry.
  2. He is nonsense!
  3. He is a nonsense!
  4. He is absolute nonsense!
  5. He is an absolute nonsense!
    Please correct all.
    Please confirm whether ‘nonsense’ is uncountable noun or not.

Nonsense can be countable or uncountable, but it is not usual for a person to be described as either ‘nonsense’ or ‘a nonsense’.
He might be talking nonsense, but he would not be nonsense.
That is a nonsense - when speaking about an action etc. is possible.