In the first movie, Simba is running away from his duties as a king, but cannot do so. In the second movie he tries to perform his duties as king by imitating Mufasa, but also fails to do so.
Mufasa himself is a majestic character, and it is easy for us to see him that way because we don’t see much of his character development. He is similar to Lady Galadriel’s character in The Lord of The Rings: both perfect and wise. This characterization was successfully achieved because we didn’t see their character development, and their presence in the films come in brief snippets. Mufasa is also given a voice that makes every thing he says sound epic.
These reasons are also the reasons that Simba does not seem as great as Mufasa, even as a father in the second movie. The audience has seen Simba grow and knows all his flaws and strengths. Not only that, he has a strong presence through out both films.
That is only why he fails to capture the essence of Mufasa to the audience. Simba sees Mufasa the same way the audience does: he missed out on most of Mufasa’s life. When he tries to imitate Mufasa in the second movie he fails to do so because he only knows the majestic side of him. Simba also spent most of his life away from the pride-lands. He knows little about it, and while he is ruling he is trying to use what he learned as a child and make it fit. As a result, he gets repetitive. Kiara was about the same age as Simba when the audience hears about the circle of life. In the first Lion King, it appears as if Simba is learning about it for the first time. In the second, it is obvious that Kiara has heard about it multiple times before.
The biggest difference between Mufasa and Simba is that Simba is still learning to become a king when he was supposed to have already known; this was a result of being away from the pride lands for so long. Mufasa was a full blown king because he had been trained to since throughout his life.