Question

Why is Marie surprised that Mersault’s mother died the day before?

Answer 1

Perhaps because he seemed very indifferent about it.
His mother died, and he continued leading his life as if nothing had happened. He ate, went out, hung out with Marie, even though his mother had just died the day before. She is very surprised about his behavior (as is everyone else) which is one of the main topics of this novel (The Stranger, by Albert Camus).