Personally, I don't really find the overprotecting of Simba with Kiara a bother. To me, there had to be a time where Kopa was around before Kiara showed since at the end of TLK, the cub in that scene is nowhere close to what Kiara actually looked like. Also, when you go back and watch the presentations, they're different. >.> Or so I think they are.
I can understand Kiara's point of view and any normal cubby will find it annoying but Simba is indeed the father and he just wants what's best for her. Another thing that might add up to his overprotection is the fact he dealt a horrible experience since he was a cub when his father died. That had to be a devastating moment for him. I'm sure Simba didn't want Kiara to do what he did as a cubby and venture off too far.

Either way, Kiara turned out great regardless. ^^