Ned is an interesting character. He's lovable because he's honorable, but he's prone to worrying about a lot of things, and never feels like he's ready for the stage he's on. He wasn't ready to become Lord of Winterfell, because that was supposed to be his brother's title. But he did it, and did it well. He wasn't prepared to be Hand, because that was Robert Arryn's title. But he did it, and did it well. But it's like he's constantly getting into things he doesn't want, and that reflects in the picture.
T'Challa, on the other hand, has been groomed for this job. He was a prince, and princes always know they are going to become kings. Unlike second sons like Ned. I think it works fine for T'Challa to be instantly self assured. It kind of worked in Avengers Civil War. He basically resolved to kill Bucky, and it wasn't an idle threat, he was pretty self assured that he was going to do it, and chased it down doggedly. So when he makes up his mind, that's what you see. Does that pan out well for interesting character development? I think it's fine. It's like Batman. He usually portrayed as instantly comfortable and self assured, even during his origins. He's fueled by his passion, and so is T'Challa, in a manner of speaking. He believes Wakandans are exceptional and that as their king, he is exceptional.
But I am probably wrong .*LOL* They have Angela Bassett playing his mom, right? There's no way that they are going to waste her talents and have her cheer T'Challa on. You can expect that there will be at least one scene where she confronts him with the reality of leadership, and what his father had to go through, and that will be kind of humbling enough.