Agree. I think people overate him on defense because of his athleticism. Like you said, he seems to be struggling lately. According to NBA.com for the month of December he's allowed his opponents to shoot 44.2% from 3 which is 8.7% higher than their average. Now compare that to November where he allowed opponents to shoot only 29% from 3 which was 6% lower than their average.
Now their probably is a bit of luck involved when looking month to month and when you take into account that the Raps defense in general has been a lot worse the numbers will likely be higher, but I don't think it is unreasonable to say JJ has been a lot worse defensively lately.