Its funny, I don't see it as theatrical, I see it as charismatic. He lights up a room with his presence and energy, he commands screen presence. And his lines well written and had weight to them, he never devolved into generic supervillain "I WILL DESTRROOY YOU WONDER WOMAN!!" territory. Thats theatrical to me.
He was a loveable evil bastard. Which is why he was an awesome villain, of the best. My only nitpick is I don't think the movie quite nailed his motivation 100%. They got the point across adequately, but I think the movie could've done a better job at making us feel sorry for him and maybe, even for a moment, side with him, like Magneto or Loki, or Harvey Dent or Doctor Octopus. I never once felt bad for the guy, he kept pontificating that he was trying to fight for the oppressed and yet, without hesitation killed his own brothers and sisters. Magneto was an extremist but he genuinely cared for his kind and would lay his life on the line if need be, I don't think Killmonger was nearly that selfless.