Many comedians, movies, authors & famous people have used this expression. However, the usage of "just sayin" as a way to absolve one's self of blame or to mitigate responsibility for any insult, was made enormously popular by the television talk show host..... Conan O'Brian. I fargin despise people who use it and/or think it's funny.
My deep hatred focuses not so much on the phrase itself, rather more about how lame, unfunny and effeminately milk-toast vanilla modern-day comedians have become. I guarantee if I heard the late, great George Carlin use it in his act.... I'd think it was hysterical. I highly doubt he would, though. Unless he was making fun of those who do.