For all I know, relationships come in all different ways. Let's see how I would explain:
Kiara and Zira: Well, at the time that Kiara saw Zira for the first time, Zira was finished speaking to Simba and does her evil chuckle and even looks right at her. Kiara seemed very aware and frightened by her when she was just a cub. Their relationship wouldn't work out, since Zira is the antagonist here. As for when Kiara is an adult, Zira holds her plan to use Kovu's relationship with Kiara in order to get close to Simba and kill him. If Kiara and Zira meet again but before the end, Zira wouldn't really be having a bonding relationship with her. She would be thinking in her head constantly "The daughter is the key to what I want. I hope my plan is working." Kiara's love relationship with Kovu is actually real and has a big effect on the plot, which is necessary. Zira doesn't have any bonding relationships in the movie. She used Vitani and Kovu for her plan and Nuka was just treated like nothing towards her. For the lionesses in her pride, they kept following in Zira's dark evil ways, but all they wanted was a good life. Zira was depriving from that happening. Kiara does offer to help Zira from falling into the river, but Zira refuses to accept help from a Pridelander who ruined what she wanted. She couldn't let go of her hate towards the Pridelanders and there wouldn't be a lot of time for their relationship to be something during that moment, since Zira dies shortly after that.
Kiara and Nuka: These two never came face to face once and their relationship wouldn't effect the movie that much. I think that Nuka's thoughts about Kiara would be biased because he wasn't part of Zira's plan to get Simba dead. Also, Nuka just wanted to be a leader with chance and not much of a relationship. He struggles with gaining Zira's attention to him. Vitani was in her plan and would act a bit more threatening towards Kiara (since she was part of what her mother really wants). Nuka wouldn't have too much effect on Kiara and he would just think "Kiara's alright. Not much."
Kiara and Vitani: Kiara and Kovu meet each other for the first time when they were cubs. Vitani wasn't there while Kovu was. If Vitani was there, she would just be on her brother's side and act means towards Kiara along with Kovu. After the crocodile attack, Vitani could have talked to Kiara for a bit and start a small relationship, but in the movie, she wasn't there. At the end, Vitani turns down her own mother and switches prides. There wasn't talking between her and Kiara and it would've not add too much to the ending.
Kiara and Zazu & Rafiki: These relationships don't really have a special point in the movie. For Kiara,they're just friends of the family and that's all.
Kiara and Nala: Of course, the movie focuses more on Simba and Kiara, which is the father & daughter relationship we see. Simba is the main character in the TLK universe and the fans wanted to see how his life would be like having a child while being a king. Sometimes, mother & daughter relationships are very different from father & daugther and father & son relationships. In TLK1, we saw the father & son relationship, which was Cub Simba and Mufasa. Nala's screen time with Kiara does seem very short and limited, but at least she gives Kiara confidence. The mother & daughter relationship is not seen in SP too much, but it is better than not having Nala talk to Kiara at all or even exist.