Question: In the *13 Reasons Why* episode where Hannah kills herself, that particular morning she wears a scarf along with a blue jacket. Earlier, when she wore the same jacket, I don't remember her wearing that scarf.

Is the presence of the scarf supposed to mean something?

Answer: 
After the rape, she must have started to be insecure about her body and chose to cover it as much as she could. So hence, the scarf.
The state of a person's mind also determines his/her choice of clothing. Given the situation Hannah was in, people tend to wear more clothes because they feel comfortable with being more covered (the connection to the mind) and also the fact that the body temperature decreases when you are afraid or are fearful or tensed about what you are going to do (the physical connection).