While I do get that there are plenty of ways to explain Kiara's out of character behavior, I still feel like they should have made her resemble Kiara from TLK2 a bit more, if not completely, and that it wouldn't have taken a toll on the plot. That's just my opinion, though.
I understand that they wanted to make her into a prissy big sister in order to help develop Kion's character, and make the contrast between the two sides, as Kiara is probably going to be a sticker to the rules along with Simba, while Kion is going to 'break the traditions for the better'. It's there to teach children not to be afraid to try out new things, but was Kiara's character change really necessary to begin with? I personally feel that Simba would have been just enough for the 'go with the traditions' side, because, as Kion's father, he is going to be a very big influence on him and it's going to be hard for Kion to go against his father's wishes.
I honestly feel it would have been better if Kiara stayed that sweet, naive but compassionate lioness we got to know in TLK2, and if she actually was one to support Kion's new ideas rather then bully him a bit. If they really wanted Simba to have a little helper on his side, they could have thrown in a new character, or used Zuri or Tiifu for the role. I believe that, by changing Kiara's character, they created a pointless small plot-hole. Because, when Kiara grows up in TLK2, she does grow a backbone, but she still remains kind and compassionate, and while we can find a way to put that together with TLG [she was a teen, sibling rivalry...] I don't believe it was necessary.
One thing I would really, really like to see from Kiara in TLG series, which is not going to happen, is for her to actually be like Mulan is in the beginning of her story - desperately trying to fit in and in turn becoming someone else. For example, there could be an episode where Kion remembers how Kiara used to be all playful and kind when they were younger - and then suddenly changed and became the perfect princess. She and Kion could have a heart-to-heart talk, where Kiara confesses that she is only trying to find her other side in being a future queen, and that it's not helping her. So then Kion reassures her that she's going to find her other side one day, and she looks away in the distance, like in TLK2. That would make me happy, but I'm well aware it's not going to happen so... oh well.