It wasn't intended to be a boy's adventure series. But sometimes intent is partly subconscious. Even when I saw the first movie in theaters--I was comparatively young at the time--I was struck by the number of elements that seemed to be geared toward children. The almost comical alien figures, using a name like "wookie" for an alien race, the cuteness of Yoda, the truncation of Luke's grief for his lost family (which seemed to take about half a scene to disappear completely), the droid comic relief. I'm not saying any of those things are bad--but they aren't exactly Joseph Campbell, either. The content does come from Campbell, but the tone is something else entirely.
Have you ever really studied mytho-poetic literature? There are some...well...comical figures in almost every culture. For example, meet the Kappa, a froglike creature that sucked a magical, not-actually-existant organ out of your anus, but was so fixated on politeness that if you bowed to it, it would bow back...causing the magical fluid in its head to spill out and leave it paralyzed. That is only one of the infinite comical creatures conjured up by the human psyche in the quest for myth. Myth and folklore are overrun with ridiculous, comical creatures and races. Why should "alien" races be any different?
Also keep in mind that, with the original trilogy, Lucas had to work with the special effects of the time. I doubt that was a willful attempt at childish comedy, but working with what he had.
Yes, I have studied mytho-poetic literature in some detail. Sure, there were comic figures in almost every culture, although some of them may not have been intended as comic by the societies that produced them. But they do tend to be related more closely to other aspects of myth besides the hero tradition. For instance, the Kappa is a trickster, and tricksters are more likely to be comic.
However, later societies virtually always adapt earlier traditions to their own cultures, so the question isn't so much what the original society intended as how a later society or author interpreted it. Case in point: The Kevin Sorbo Hercules series. (I think the name was
Hercules: The Legendary Journeys.) The Herakles of Greek myth was a heavy drinker, an epic fornicator, and someone who killed indiscriminately (though even the myths vary in how he is portrayed). In contrast, Sorbo's Hercules never drinks, never has sex outside of marriage, and never kills human beings. (The story in which Hera drives him insane so that he kills his own children is modified so that Hera does the killing directly.) And then there's the Disney cartoon
Hercules, in which Hercules, rather than being the product of one of Zeus's adulterous relationships, is the son of Zeus and Hera. He's separated from them, and grow up among ordinary humans as an awkward teenager.
The adaptations above are so different that if the names were changed, people could not readily recognize them as the same story. I would argue that Lucas's situation is analogous. Yes, he is consciously following the idea of the hero's journey, but the characters are composites from various cultures rather than being adaptations of particular mythological figures. As such, it is Lucas who determines how the different components are mixed together. In particular, he determines the proportion of humorous material, which, however it might have played in earlier cultures, is more likely to appeal to children (or to our inner children) in our society. In other words, our modern cultural expectations are at least as important as the underlying myths, if not more so.
Nor is this kind of adaptation of originals somehow unique to modern society. It's been happening since the very beginning. Herakles was originally a more heroic figure. When the Dorians invaded Greece and claimed that their leaders were descended from Herakles, the native population responded by creating or adapting the myths to portray Herakles in a less favorable way. Odysseus is portrayed as noble in Homer, but later portrayals make him more and more deceitful and cowardly as time goes on. (The Romans, who thought themselves descended from the Trojans, encouraged this development.) When Josephus tried to present the Torah and other Jewish sacred works in a way that Romans would understand and appreciate, he frequently changed the story significantly. For example, Moses is originally portrayed as fleeing Egypt after killing an overseer who was abusing a Hebrew slave. In Josephus's version, Moses flees Egypt because he is so handsome and talented that everyone envies him and wants to kill him. Early Saxon converts to Christianity had a hard time figuring Jesus out, so when the
Heliand (Saxon Gospel) was created, it portrayed Jesus as a war chief and the disciples as his sub chiefs.
Yes, Lucas was consciously drawing from myth--but he was influenced by contemporary culture as well, and that also provides insights into what he was trying to do.