What is certainly odd is Meursault's lack of a reaction. Upon hearing news of his mother's death, his is dispassionate, nonchalant, as if he's heard of a poor weather forecast for the upcoming weekend and that it may inconvenience his plans.
Mother died today. Or maybe yesterday. I don't know.
Meursault is only affected by his immediate surroundings, his present environment. Since he does not interact with his mother on a daily basis, her passing does not affect him in general. In fact, he is mostly concerned with missing work on account of the funeral. Meursault is comfortable with the repetition of his daily life. His mother's death, although it doesn't seem to affect him much, is the first of a series of events which disrupt that repetition. He then is conflicted with conformity vs. free will or social conformity versus individuality. Philosophically, Meursault is conflicted because he has no belief in abstract ideas. Thus, his mother's death, and memory of her, does not affect him as much as it should because he's developed this meaningless view of existence; he believes that only immediate things affect him. Abstract idealism, human condition and so on have no effect on Meursault. He is a nonconformist but this leads to a degree of self-centeredness that leads to his downfall.