Yeah, he certainly didn't adjust but there were confounding factors in there: he stopped hitting the ball as hard as often after the concussion, full stop. That was the biggest cause of his decline.
Second, the Twins franchise at the time simply was not capable of steering anyone in the right direction, as they were as far behind the game as anyone. My god, look what they did to Hicks and then Buxton.
It feels like there was a significant portion of post-concussion Joe Mauer performance left untapped by a franchise that didn't have the coaches to identify and fix holes the modern game was exploiting.