Superman is, historically, the worst at disguises. In real life, combing your hair differently and putting on glasses wouldn't fool Ray Charles, let alone the world, but that's part of who he is, so we're all okay with that by now. Somewhere along the lines, live-action adaptations of Superman decided that he also needed to add plaid to his repertoire of implausibly common wardrobe choices. Maybe it's because he grew up on a farm in Kansas, and that's what farmer's wear?
Whatever the reason, ol' Clark's been putting on plaid whenever he visits the family farm ever since 1978's Superman. It's usually red, but not always. It was white in Superman IV, and white again in Superman Returns. Man of Steel got nervous and put his mom in plaid, but, oh, there's Clark wearing plaid again in Batman v Superman. All the way in Metropolis too. And who can forget the CW's Smallville, a show that spent more on plaid than Sister Act's entire budget for nun costumes.
Which brings us back to Justice League, where Clark, grumpy after getting woken up from the dead, heads out to the farm in Smallville to talk to Lois. In plaid. Even though she totally knows who he is, and he can just wear the suit with the big S. We mean, she just called him Clark in front of a dozen policemen while he was Supermanning the Super Friends. So why did he change to plaid? Because it's an Easter egg, is why.
Look, the bottom line is: if you ever see a guy anywhere wearing plaid and glasses, there's a 30% chance it's Superman.