Punk didn't cause smarks to turn on Reigns. You could see the boos starting in Summer of 2014, they were pushing him over Ambrose (who was loved by smarks), and his push was very reminiscent of Cena's. But Punk did him no favors on that podcast either.
This is only a half work I think though. CM Punk seems angry all the time and like he believes his gimmick is and needs to be an 100% shoot.
I don't think Punk himself caused the turn, because the way Vince was setting things up (You know damn well the SS2014 moment with Sting was supposed to be Roman's moment before the injury) all but guaranteed Roman would get the Cena treatment at some point down the line. But Punk's comments definitely exacerbated the shift in perception from the fans and made that shyt happen ahead of schedule. Because this wasn't JUST coming from a jaded ex employee. This was coming from the guy who a vocal section of the fandom saw as the 1B to Cena's 1A for a solid 2-3 years prior and could do no wrong.
Rather than a gradual period of "Okay, he's nowhere near as good as they're making him out to be and still learning on the job, but they haven't fukked him up YET" we got with Cena before the mega push, These comments along with Vince's heavy-handed booking pretty much left no chance of him getting a fair shake at seeing what he could do and would take 3.5 years and a real-life illness to reset.