Guess it depends where you look.
According to PFF, his YAC was 2.8. It wasn't his lowest career YAC. That would be the 1.9 he had in 2015 when he had, arguably, the best season of his career.
His career average is 3.5, so you're talking about an extra 40 yards of RAC. So, like, 2 or 3 extra broken tackles. Considering he was suspended for the first six games, the sample size is super small and really not all that relevant. Not to mention 2 of the last 3 seasons saw the Cardinals running his shortest yards per route in his career. Considering he doesn't run a ton of option routes - I assume, anyway, based on Murray's skillset - that means its schemed by the team and called during the game, not because he can't run them. Calling curls ain't going to get him the YAC.
He had four total games with Kyler Murray last season. The passer rating when targeted by Murray for those games:
In the six games with Colt McCoy and some guys named Trace McSorley and David Blough throwing to him:
His career average is 97.9. He blew his average out of the fucking water when he didn't have 3rd stringers and journeymen throwing him the ball. Why do we really think his season numbers were low?
Neither of your metrics actually show that, and nothing about what I saw from him the few times I got to watch him said it either. He's 30 years old and isn't going to be 1500+ yard DeAndre Hopkins, but the guy is still a talented, legit #1 WR and should be for another few years.