I'd say the smile is typical Johnson. He did the same against Burns and taunted him throughout the fight.
I think that the smile and taunting was all part of Johnson's game. It helped to strengthen the impression that he was controling them with ease, took their confidence and sapped their will to fight back. He didn't necisarily do it to humiliate them in all cases.
Jim Corbett smiled as he punished his opponents as well. In those days Corbetts smiling was playful, while Johnsons was malicious lol