That seems unlikely; Hero With a Thousand Faces and Campbell’s later Masks of God series were standard reading for creative writing students (despite points of dispute about Campbell’s overzealousness to make all myths fit the formula) and Lucus would certainly have been at least introduced to Campbell at USC Film School. I think much of the “greatness” of the original film is owed to the fact that it is largely a homage (or less charitably, a blatant copy) of scenes and characters from other films and stories, most notably The Hidden Fortress, The Dam Busters, 2001: A Space Odyssey, and The Guns of Navarone, as well as significant elements from Le Morte d’Arthur and Dune. Borrowing from great source material does not always make for a great story, but in the case of Star Wars a blender approach with much of the guidance and input Lucas received did result in a sensible combination of classic elements.
The film may not be great compared to modern standards–I don’t think it has held up nearly as well as Raiders of the Lost Ark or Alien–but by comparison to films of the day, and especially science fantasy, it was head and shoulders above the rest. Just watch Logan’s Run, released the previous year to acclaim of its special effects, and compare the two films; in every aspect Star Wars is clearly better. The sets and location shooting–using exotic, difficult to film locations–made it look truly unworldly compared to the variations of Southern California desert that were the typical outdoor shooting sets for films set on alien worlds, as were the seemingly large, intricately decorated interior sets. Virtually every science fantasy film that has come since owes some aspect of set design and staging to either Star Wars, 2001: A Space Odyssey, or Forbidden Planet.
The clarity of the story and distinct character motivations may also be some part of the appeal, especially to younger viewers uninterested in political machinations and double crosses, but The Empire Stikes Back did quite well with both of these and a bifurcated story line, so I don’t think that can be considered the critical appeal of the film except in comparison to the convoluted and borderline unintelligible storytelling of the prequel films. The story in the prequels isn’t really all that complicated but is told in such a way–presumably in an attempt to introduce some mystery about who is playing whom–that renders it confusing and frankly uninteresting as a motive structure to advance the story. This, combined with special effects that were elaborate but about as realisitic as a cartoon and flat, unaffected acting, made them poor films not withstanding the essential lack of necessity in telling a backstory and constructing rationale for they mythological elements of Star Wars. Everything that was unique about Star Wars, including set and sound design, music, costuming, elaborate motion control scenes with massive space battles, had been copied so much by successive films and imitators that the effort to top the original effects and setting in the prequel films made them overstuffed and underwhelming.
In essence, what made the original film great, as already susincctly noted by Chronos and madsircool, is that it was a simple ‘B’ movie story done with enthusiasm and effective setting. The special effects were fine for their day, but it was really seamless the flow of the film and eschewment of narrative excess that kept it moving past some of the clunkier bits. That, and Han Solo, who clearly is out of his depth but manages to save the day repeatedly despite his Jack Burton-esque near ineptitude.