What theme is once repeated in Hamlet's rhyme "imperious Caesar dead and turned into clay/might stop a hole to keep the wind away."

Throughout this play, Hamlet has contemplated death and
what happens after death.  He contemplated suicide, but worried about God's command
against it.  He contemplated suicide in general and realized that no one ever returns
from the dead to tell us, the living, what it is like, so the fear of the unknown, keeps
us all from killing ourselves as a relief from the burdens of life.  By Act 5, he has
managed to escape death by re-writing the letter to England, and he is now literally
standing in a graveyard, talking to the grave digger whose whole life is about death. 
What he is asking in this quote is a rhetorical question that implies that Hamlet
understands that death is the great equalizer and that death is kind of meaningless.  He
is alluding to the old expression that death means "ashes to ashes, dust to dust."  That
we come from dust and return to dust after death.  No matter who we are, or what we did
in life, it all doesn't matter after death.  The lowly beggar and the great Julius
Caesar will endure the same fate -- dust.  He is saying that even the great Caesar is
going to be dust that might be used to create a mud paste that will keep the wind out of
a house.  Once Hamlet realizes that death is kind of meaningless, it frees him to act
and do what he needs to do in order to avenge his father's death and take care of
Claudius once and for all.