Comics were no exception. While artists at Marvel, DC and smaller companies were experimenting within the established genres, mainstream comic books were still shackled by the public's expectations of them (not to mention the very restrictive guidelines of the Comics Code Authority). The very fact that comic books today are still considered juvenile entertainment can be traced back to the self-censoring attitudes of the big companies; while the world around them was changing rapidly, the comics of the day seemed almost quaint. Change in the business meant giving the teenaged sidekicks a happenin', 60's kind of lingo... not pushing the boundaries of what a comic book could be. No, it was business as usual for the big businesses, and there wasn't much reason for them to believe anything would change that.
Then came Zap.
Zap Comix wasn't the first, but it was the most successful of the 'underground' comics to come out of the 1960s. The same spirit that encouraged actors and artists to try new things extended to cartoonists, many of whom wanted nothing to do with the mainstream comics industry. Compared to mainstream comics, their comics were infantile, scathing, sexually suggestive, sometimes crudely drawn and almost always grounds for punishment if your parents found them under your bed. Most people, sad to say, just weren't ready for the idea that comics could be something you'd have to think about.
But despite - or, perhaps, because of - the disapproval of authority figures, the undergrounds were as important to the industry as anything that came from the 'official' publishers. First, they capitalized on the fact they didn't have to answer to the Comics Code Authority and used their pages to tackle contemporary topics in scathing, often obscene, ways. By doing so, they showed everyone that comics didn't have to be just about superheroes or funny cartoon characters - they could be about anything, including real life.
Second, they brought a new style of storytelling to the page. Instead of the straight narrative style used by practically all the books of the time, the undergrounds led by Zap popularized stream-of-consciousness prose, dream sequences, breaking down the fourth wall (characters talking to the readers), and just plain absurdity. Later artists, underground and otherwise, would owe a huge debt to their experiments.
Finally, Zap Comix introduced a man named Robert Crumb to the masses, many of whom can safely say they haven't since looked at sanity in the same way since. Crumb is often considered the greatest of the underground artists, and his work certainly embodies all the irreverence and wit that the best tried to capture. Many of today's comic artists and comedians consider him a huge influence on their work.
Zap Comix only lasted for 13 issues, but its reprints ran into the millions of copies, and its impact on the comics industry was significant. Finally, here was a book that refused to fit into any preconceived genre, and it broke open the floodgates for self-publishing artists who couldn't or wouldn't break into the industry by going the normal route. Crumb and his contemporaries brought a level of maturity to the business by reveling in the silliness of it all... and if seeing the words 'maturity' and 'Crumb' in the same sentence seems baffling to you, just imagine how Crumb would feel.