At the beginning of STID, the writers definitely regressed Kirk's character because they wanted to give him the story arc of re-earning the captain's chair. The party line that I heard in several interviews was that Kirk got the captain's chair in the first film, but he truly earned it in the second film.
And to be fair, there was a lot of criticism after XI that Kirk had become captain too fast, so the writers were trying to address that. But IMO they laid it on too thick at the beginning of STID. Kirk starts the film by accidentally stunning his and McCoy's "ride" leading to the crazy jump off a cliff, and then we get Scotty chastising Kirk for putting the Enterprise at the bottom of the ocean. We also get a line from Sulu when the shuttle is failing, where he says something to the effect of, "I told the captain that the shuttle wouldn't be able to take this." Followed by Kirk's nonchalant, "So they saw us, big deal." Then of course the threesome with the Caitians, and it's also strongly implied that Kirk had a fling with Christine Chapel and dumped her.
So yeah, the writers went overboard with the immature playboy stuff in the early part of STID, but it was all in the name of giving Kirk a character arc where he could grow up (having Kirk's surrogate father die was part of that, too) and truly earn his position as captain of the Enterprise by the end of the film. I have faith that Kirk will be more mature in the next film, especially since I do expect a romance with Carol Marcus is in the cards.