The teams they played did play a part in who looked good and who didn't. I would also say that with Napoli and PSG having new managers, and then Liverpool working on a new system, it did impact a lot of what we saw in performances.
I think, and I certainly think it's more than fair to do so, that part of the trouble with giving a ton of credit to PSG is related to the amount they spend, and the level of quality they have. They have more money than any club, they have two or three of the best players in the world right now (probably another 2-3 of the top 15-20), and they shouldn't really struggle to go through. (edit, I don't know if struggle is the right word, but they definitely made it harder than it should have been, tough group or no) They made it, they won the group, and that's what matters, but I don't think they played to what they are capable of. I certainly think that's a bit overblown, as no one will care that they didn't play well if they win the title, but did their group play or past history give you any faith that this year is different?
I do think Liverpool messing up and not winning the group through mistakes is a bigger story, considering they should have done better.
I also think some of the trouble is that going to Red Star is a lot harder than people think and give credit. Napoli is also tough to play against at home.