I agree, she is extremly immature, I am not even that immature and I'm 15. I belive that Rikku is meant to be childish, there is always a quiet/shy character, a rude/mean character, and a outgoing/loud/immature charcter which in this case is Rikku. At times I find her extremly annoying like when ever she speaks, but she only annoying to me when she talks, other wise I can tolerate her.
