Real life isn't pretty like your typical Hollywood film would have you believe, and American Splendor, both in the form of this film and the comic book, illustrates this fact in a way that we don't wind up walking out thinking, "Well, duh, life sucks - we all know that." The bigger story at play is that people find each other and find things that redeem them from the doldrums of everyday life. Pekar couldn't draw but had a story to tell, and finally figured out that through comics he could present life in all its ugly beauty. His stories saved him at a point when he had nothing else to lose. That he never makes a commercial success out of his work even after national exposure from David Letterman is exactly the point - Harvey does what he does purely because he has to, because it keeps him as close to sane and normal as he'll ever get. This point couldn't be rendered more perfectly than when a playwrite undertakes the task of presenting American Splendor for the stage. Harvey and Joyce's first date is, in real life, a butchered mess that ends with her vomitting over and over in his bathroom, but this scene is butchered in a typical, sacharine-sweet and entirely predictable manner for the stage, completely missing the very point of why these two would have to get together in the first place - no one else would possibly want to have them. Just like real-life, their love and life together is odd and awkward, and rather than learning to accept each other for their faults, like every typical Hollywood movie wants you to think, real people like Harvey and Joyce simply learn to live around them.
(You'll like unproductivity.)