What is Hamlet's state of mind when we first see the ghost? Why does he feel that he is "too much i' the sun"?

Your quote is actually mentioned before he sees the ghost (act 1 sc 2). Hamlet is using a slight play on words. Claudius has just asked Hamlet why the clouds still hang about him (why is he so sad). Hamlet replies that he is two much in the "son". Hamlet is being sarcastic about his new relationship with his Uncle Claudius. Two months ago, Claudius was Hamlet's uncle and now Claudius is his step-father. Hamlet implies that he does not want to play the role of a son to Claudius. Hamlet is curious and excited when he first beholds the ghost.

No