No, he isn't. J.R. tends to go in when defending star players but struggles and doesn't have the same level of engagement against lesser players, there's been articles written about this and I've noticed this since he was a Knick. Also, he is undisciplined in terms of silly fouls whether running through screener, fouling three point shooter, or bad reach-ins. I don't think any metric rated J.R. Smith as a + defender last year.
As much as y'all like to get on D-Wade for not running back in transition against Boston, J.R. was out here doing this
D-Wade still has strong defensive instincts and physical tools, he can still jump in the passing lanes, he can still block shots, and he can still dig-in on the ball in those tight spots.