You don't think critics calling him a hack, Rolling Stone saying his screenwriting would induce projectile vomiting, saying he has nothing of substance to say, that he his films are not worth your time, that he is a greedy businessman trying primarily to sell toys and not make art--you don't think that would be seen as something personally offensive to him? If that isn't, I don't know what would--newspaper critics aren't going to tell him to go jump off a bridge or anything that harsh, and most people online aren't that harsh too, they mostly just stick to trashing his attempts and critizing his motives and saying he is greedy, sell-out, etc., pretty similar to what some of the harsher proper-media says.
And, I also don't think he is talking about any one group of people in that quote. He doesn't say "fans" or "audiences" or "people on the internet" (a group he has specifically called out for their complaints on more than one occassion). Critics in newspapers trashed him and his films. Fans on websites and forums trashed him and his films. General audiences had some criticisms for him. And people writing reviews and pieces online trashed him and his films too. But these are all "critics", they are people giving him criticism, not just newspaper writers--the internet changed a lot of things compared to 1983. Lucas knows all of this because he has shown that he is well-aware of these activities--he's even made an ambiguous possible reference to OT.com in the last months.
So that's why he says "everybody" criticized the films. He's right. His critics weren't just those writing in newspapers, there is a whole other world where people have a publishing medium for criticism, with actual print media just being one component of a much larger picture of widespread criticism. Fans, audiences, newspaper writers, critics online and in websites. That's why he doesn't say "online fans" or "newspaper writers", he says "everyone", he knows that there was pretty widespread and wide-ranging criticism and he knows it wasn't confined just to print media, especially today. And, of course, all that criticism does hurt on some level, because people are saying not only are his films not enjoyable but that he has no talents. I'll re-quote the 60 Minutes interview from 1999:
Leslie Stahl: When critics go after your directing, your writing--it has to hurt.
George Lucas: Oh it hurts. It always hurts. It hurts a great deal. But part of directing is that you get attacked, sometimes in very personal ways. [goes on to his green-house/white-house analogy]
So, yes, while I don't think he necessarily thinks critics are important, he clearly doesn't think they are unimportant, and the picture you are trying to paint about him being stoically unaffected by critics is not accurate. Clearly, criticism from people, whether they are writing in a printed magazine or simply posting stuff online, is important enough to him that he is using it as an explanation for his retirement. Whether you believe that or not--I don't really--the fact that he would state it in the first place shows how much the criticism irks him personally.