why does this act start with a grave scene? What attitude toward death does Hamlet reveal as he speaks of Yorik's skull and of Alexander as a plug of earth "to stop a beer barrel"?

There is a bit of comic relief between the two grave diggers. This breaks up some of the tension that has been building up. We get some witty high brow banter from some lowly grave diggers which constitutes most of the humour. Hamlet takes a look at the skull of Yorick, the court jester. Hamlet starts waxing all nostalgic about him and transitions into an important theme of the play; we all end up dead. From king, to lawyer, to jester to beggar, people end up back in the dirt. Even Alexander the great is used to "to stop a beer barrel". This is everybody's destiny. No man no matter how great escapes the ultimate reckoning. This is a destiny our physical bodies all share regardless of God or the afterlife.