There's even a letter from her mom in it!
“The worlds been hard on us…hard on him. Joel’s done some terrible things. He tells me that on this journey, you either hang on to your morals and die, or do whatever it takes to survive.”
— Ellie (The Last of Us)
Yes, and I believe at least some if not all Ellie's items get counted as collectibles. Later if you count the items in Joel's backpack the number falls short (by 9 maybe--not certain of the number now) of the actual number listed in the Stats. In other words, I'm not certain, but if you don't look through Ellie's items in her backpack you may not be able to get the collectibles trophy?