Look, people. Rikku isn't really as immature as she acts; she simply is trying to make everyone feel better about themselves. If you pay attention, she acts less mature in FFX-2. Wanna know why?
1. We first meet Gippal at Djose Temple in the beginning of the game, an inquired past love of Rikku's. Notice how immature she begins to act? She's not sure how Paine and Yuna would take it if she told them about their past.

2. Yuna is so freakin' depressed during the whole game that I honestly wanted to shoot her. Rikku gives the game a more positive feel with her happy-go-luckyness. It doesn't help that Paine is just as anti-social as Heero Yuy from Gundam Wing...

My basic overall idea is that just because she acts the way she does doesn't mean she's mentally retarded or anything...she does it to cheer Yuna up. I tend to be the same way, does that mean I have mental issues? CERTAINLY NOT! I'm 14 years old, with the maturity of a 16 year old, as I have been told by several adults/peers. I act childish to cheer people up, or even, when I'm being self-conscious about something, I get that way too.

About the way she dresses:

I guess the fact that she flashes more skin and wears brighter colors shows that she may act immature but she's got the mentallity of an adult. I mean honestly, if you saw a teenage girl wearing something similar to what Rikku wears, what would be the first thing on a teenage guy's mind? Perverted things, I'll tell you that much. Rikku just wants to show that she can be sexy and funny at the same time.

~*~Celes~*~