The Fleischers were just so abstract and surreal, and it gave a great unique quality to their cartoons, especially the Popeye's. And what's neat about Popeye's mumblings is that they were all improvised. In those days, the Fleischers recorded their soundtracks AFTER the cartoons was made (the opposite is done these days), so Popeye's voice artist Jack Mercer took some liberties and had the character mutter the most wild, spontaneous things. Mae Questel who did both Olive Oyl and Betty Boop did it was well with her character, and soon it became common practice with all the actors.