My guess is that most people out there can't remember when there weren't UPCs on everything, but for me and my generation it was a big deal.
Here are my three favorite comics from January 1976.
And here they are from February and March 1976.
My world would never be the same again. :-(
I would eventually get used to having the mark there. What I ended up really not liking is the Spidey head that would take the place of the code, or the DC bullet PR box ("There's no stopping us now!"). Those were on the Direct Sales comics, which I never understood why they wouldn't need a UPC, too. But...yech.
In the 37 years that we have had UPCs on the covers of comics, I have only seen ONE cover actually use the required box IN the illustration. I may have missed one at some time, so if there are more out there please let me know. In the meantime, hats off to the great George Perez for his illustration below: he has the Atom "riding" the UPC box in a very clever way.