Before you start humming The Doors' "People Are Strange," hold up. We're not just dealing with Meursault being a total weirdo; we're dealing with translation, colonialism, and existential angst.
Let's start with: "What is the title?" In case you didn't know, Camus was French; so he wrote The Stranger in French, and, because it seemed appropriate, gave it a French title: L'Étranger. Here's where things get tricky—in the translation. "L'Étranger" could have easily been translated as "The Foreigner" instead of as "The Stranger," and actually is in some cases.
Translations aside, it's more fun to argue semantics. Let's run with this "foreigner" bit. Our main character, Meursault, is a French man living in French Algiers. In some senses, yes, this makes him a foreigner to the land, but the text establishes that in fact his family has lived there for several generations—in a colonialist capacity, yes, but they've still been around. They know Algeria. More likely, Meursault is a metaphorical foreigner. We know this guy is detachment personified, so it's easy to argue that he's a foreigner to society, to common, human customs—he's an "outsider" (yet another possible translation for the title, by the way).
This is based on the word "foreigner," but the same thing applies to the title The Stranger. Meursault is a stranger among other people because he is so isolated from them—mentally, emotionally, spiritually, and, by the end of the text, physically (he's imprisoned).
He's strange. He's the strangest. He's the stranger.