It is (or should be).
* XP folk very explicitly state that you shouldn't be "doing XP" N months down the road. You should be optimising your process
* The Kanban/Lean folks whole method is about having a process for process improvement
* Scrum very explicitly has Sprint retrospective and reviews aimed at looking what happened and changing the process to improve results next time.
* Crystal is all about aligning the amount of process you do to the working context you're in.
Of all the agile processes Scrum is really the only one that says you must do certain things in a certain way - and even there it's a very light framework of five events and three artefacts) that gives you a space to figure out your own process in.