Holden's fascination with the Egyptian mummies suggests that he is curious about death. In his paper he wrote:
"Modern science would still like to know what the secret ingredients were that Egyptians used when they wrapped up dead people so that their faces would not rot for innumerable centuries." (Salinger)
Holden is still dealing with the death of his brother, Allie, three years ago in the novel. His thoughts go to death several times in the story. He considers his own death, he talks about the death of a school mate, James Castle, when he attended Elkton Hills. Castle jumped out the window, committing suicide wearing a sweater that he borrowed from Holden.
Holden considers both life and death in this novel, as he realizes that he has no choice but to grow up, can't stop the process, he considers the ramifications of death.
Not only does it show that he is curious about death as pmiranda mentioned, but it also shows Holden's obsession with the idea of youth. He mentioned the fact that he was enticed by the fact that these people could be dead for so long yet still look so fresh and almost alive. Throughout the book we learn that Holden sometimes wishes he could freeze life and keep going with his youth (museum of natural history scene), the mummies symbolize this idea.