[stinkytaco]

As already pointed out, he was a drug user ("I like the Fed-Ex guy because he's a drug dealer and he don't even know it").

But the delivery is his style. Whether it's natural or not I don't know, but it certainly was an integral part of his comedy, similar to the monotone of Steven Wright (whom Mitch Hedberg is the spiritual successor to, in my opinion).

EDIT: Curiously, Hedberg suffered from terrible stage fright. I find him funny to listen to, but almost uncomfortable to watch because he would often stare at the stage or close his eyes.