To clarify this story (it doesn't seem anyone else has mentioned this yet) according to other sources talking about this Shia clarified his retirement as being from social media. Meaning he's no longer going to be running twitter feeds and things like that and monitoring them.
Apparently, his side of the story is that he never actually confessed to plagiarism, and his comments had been taken out of context or edited. His use of intentionally plagiarized apologies, which were intended to be recognized for what they were, was an attempt to mock the guy bringing the suit against him, but it wound up backfiring. He apparently took his stunt to the point of hiring a skywriter to say "I'm sorry" as a joke, but then got called a sociopath for doing so. In short he tried to rally internet support with some amusing antics, trying to make his accusers look ridiculous, and it backfired massively because he just has that many haters.
Shia's antics also apparently included a faux-trailer for "Daniel Boring" which is the title of another Clowe graphic novel.
Basically the biggest epic misfire on an attempt to rally support with social media in history up until this point.
As far as the accusations themselves go, I have neither seen Shia's work or read the graphic novel, however some things I've read have said that Clowe's work is pretty stereotypical here and if you called Shia a plagiarist you'd have to call dozens or hundreds of other people the same thing at this point. Clowe's big claim to fame being that he did it as a graphic novel.
That said, while considered famous, Clowe had a niche following and was famous within specific circles. Honestly he's benefitting more from the attention here than anything. He's probably going to see more people running out to buy his work to see what all the fuss is because of Shia than he has at any other point in his career. In some ways he should probably be thanking Shia, even if he is a plagiarist, since Clowe is probably going to make some decent money off of this from the attention.
At least that's what I've been able to pick up from other articles on Yahoo, and various discussions about the subject. I don't read Clowe's stuff, and honestly don't much care for Shia, so I don't have much of a stake in it. In this case though the outright hate Shia can inspire from his franchise work seems to have turned this on it's head. At the end of the day he was stupid and tried to appeal to a fan base he doesn't actually have, due to a legal dispute that probably never would have went anywhere.