Eh, I've always considered Dirk a reasonably streaky player (basically just like any pure shooter). Unlike most players, he will go 1-15 one night and 14-15 the next night. Those will tend to push your percentages around. Last year might have been the exception, where he shot over 50-40-90 all season (though if I'm not mistaken he started the year off ~5 ppg less than where he finished).
Nothing from watching Dirk has led me to believe he's lost anything. In fact, he's shooting 47-34-91 right now. You also have to remember that Dirk just standing around the 3pt line is one of the reasons Dallas has had the top offense for over a decade. Players have to guard him differently than anyone else in the league, comparing him to Spencer Hawes is completely ridiculous. Especially considering how you can close out on Hawes without worrying about him putting it on the floor and going by you (at least anywhere near as well as Dirk can).