They have many of the same perspectives on relating to other people. Belle and Tiana offer a critique of other people when they notice other people's flaws. Belle and Tiana also constantly compare themselves to other people.

Not "good" friends but friends, yes. I think Belle is too dreamy and a bit passive for Tiana's liking and Tiana doesn't really care about adventure or romance. But they do have similar attitudes and are close to their dads, so I think they'd still like and have a basic respect for each other.

I don't get why people often see Belle as mature and serious type. Just because she reads books? Belle is relatively cautious compared to Ariel or Merida but I don't see her as too serious or mature. I wouldn't oppose her to Ariel.
I don't see them as close friends.

^ I absolutely agree with you! They are not nearly as similiar as some people say.