what frustrates death with regard to his appearence

Death tries to clarify what he looks like. It is a little humorous as Death tries to counter the human stereotypes about his appearance. He is obviously frustrated by how he is portrayed in the Judeo-Christian imagination,    

I do not carry a sickle or scythe. I only wear a black robe when it is cold....    

Zusak develops Death's character without pushing human characteristics too far for the reader to stop taking him seriously.

Why is Death overwhelmed?