I really think you're underestimating how much of a sense of humor the original film had about itself. I was watching some of it just the other week, and was struck by how much of it was an affectionate comic deconstruction of its own genre. Luke actually said he couldn't see a thing through his Stormtrooper helmet -- that's as meta as anything on Robot Chicken today. And the way Luke and Han bumbled through the rescue and then the princess proved to be thoroughly unimpressed was a great way of deflating and lightly mocking the conventions of the adventure genre.
And I should remind you that C3PO and R2D2 had more screen time than practically any other character in the first reel or two of the first movie. The comic relief was always center stage.
Honestly, I'll never understand Star Wars fans who act as though it was meant to be some great serious epic and that treating it with humor is a betrayal or something. It was meant to be a playful pastiche of Saturday matinee adventure serials. The original three films worked because they didn't take themselves too seriously, because they remembered to be fun.
I also think this had to do with George Lucas having read Mad Magazine growing up, because he hasn't been shy about its influence on his work (He wrote an introduction for a book that collects Mad's Star Wars material, and always spoke highly of their parodies of his movies).