I don't think Tarzan would count because the strip was derived from the Burroughs novels. What year did the comic strip begin? I'm certain the silent Tarzans pre-dated the comic strip.

Prince Valiant would certainly qualify and pre-dates Superman by decades. So does the Li'l Abner musical you mentioned. Both of those films were big pictures and not matinee fodder. But I think the first big budget comic to film adaptation would be Flash Gordon. Can you think of any that were made prior to Flash Gordon?