What really changed with Kupp the most the 2nd half of last year was targets. Now is that on him? Goff? McVay? Due to Higbee emerging? Impossible to say. But that is the number that fell off the most- from 87 (10.9/game, a pretty unsustainable number) to 47 (5.9/game- below his career average). Almost half.
But he did actually catch a higher percentage of his targets (77%) the 2nd half of the year vs. the first half (67%) with as many TDs (5). In my opinion he disappeared really in only two games- @PIT where the offense was awful (0 catches on 4 targets) and the next week vs CHI (3 catches on 3 targets for 53 yards- but the Rams also only threw the ball 18 times that night). And he wasn't that effective the next week vs. BAL either (35 yards on 6 catches) but again the Rams as a whole were steamrolled.
I'm not going to ding him for having three down games. For 3 years in total he's been excellent. He's not DeAndre Hopkins or Michael Thomas or Julio Jones. But he is in the upper tier of WRs in the league. I think any NFL DB would say that.