No I have not really noticed that. He is swinging as hard as ever. I am not sure he has played enough to have slumps. I don't consider 0-10 after being on the DL for months a slump.
You just make this criticism all the time against certain players but not others. It is just a lazy stereotype that paints them as resistant to coaching and is really not based on scouting. The biggest difference with Quinn this year is he is swinging more at pitches in the strike zone rather than chasing. His Z-Swing % has gone from 63% last year to 73% this year.
This has not manifested in better OBP yet, but it is a good trend that likely will if it continues. I would not say his OBP is appalling. It is .317 career and dropped to ..295 this year mainly because of the walks dropping. His K rate is down a bit too and I think this is being anxious in general. If he continues to swing more at strikes, it is likely both his BB rate and K rate will improve compared to the past.
As Zambonir says though - clearly still a work in progress. His OPS bar to clear is pretty low though (like 700) to be worth playing because of the baserunning and defensive benefits. If he just gets his BB rate back to his career rate of 8% he is then a 260/320/380 hitter. That is probably a 3-4 WAR player when you add in everything else. He is not there yet (and not obviously better or worse than Haseley also).
At the very least he has probably bought himself a few more productive years in the majors as an extra OF and pinch hitter. That is progress mainly because he is healthy and has a little opportunity (Cutch injuries, Haseley going away, Williams failing last year).
