I think that Rikku is acting less tense. in all of X, everybody feared for their lives all the time because of Sin. Sin being defeated allowed everyone to lighten up and relax. this kindof explains why Yuna's wardrobe dramatically changes too. now she has legs =P

it's also safe to say that Rikku is too relaxed. i agree she's more goofy than she was in X. maybe she's just more slutty is all. her clothes say so.