This certainly makes all the sense in the world. The Black Panther of Marvel Comics knows how to occasionally kick back and relax, but being both a nation's king and most trusted warrior also doesn't give him a lot of time to slack off. What's more, the start of this new movie will not only see him still coming to terms with the assassination of his father, but also trying to convince the people of Wakanda that he is the right man to lead them into the future. None of that is going to leave him a ton of time to come up with crafty one-liners or bits, fully explaining the more intense tone.