First, while I respect your disagreeing opinion, political discussion (and bashing) aren't allowed here Bob, so please be respectful of fthat. It's a site rule is all, so please just keep that in mind.
I'll reply with why I don't believe Joe's propoganda though...
“any organization or plan for spreading a particular doctrine or a system of principles”
G.I. Joe, while like "Uncle Sam" as a fictional character, isn't organized to spread a doctrine or system of principles. He was a fictional character (set of characters) set into a specific story fighting a specific enemy.
He was based in America, but didn't fight American enemies that are real. To that end, the only principles spread by G.I. Joe were basic moral ones. Also, G.I. joe marketed in other countries was often not marketed as "A Real American Hero", but rather marketed generically or to the country in question.
Basic moral messages of "do what is right" were the only propoganda G.I. Joe ever seemed to spread in my view. That's what I got from him as a child. I feel it cheapens a lot of people's childhoods to say otherwise, but that's just my opinion.
“a specific type of message presentation, aimed at serving an agenda. Even if the message conveys true information,”
G.I. Joe didn't serve an agenda other than his own... He wasn't aimed at any one particular "enemy" other than Cobra, a fictional one. He wasn't a recruitment tool, didn't endorse joining the military, etc...
Just because he was pro-American doesn't mean he was pro-military power to reach an end. If anything, it was the opposite in the cartoon series, which preached non-violence to a kind of funny extent when you watch the cartoons now as an adult.
The comics were more aimed at adult thinking, just as they are now, but I still don't see a correlation to propoganda for the U.S. military there.
“the spreading of ideas, information, or rumor for the purpose of helping or injuring an institution, a cause, or a person”
G.I. Joe reached many audiences, but didn't spread any anymosity in its themes against U.S. enemies of the time. For instance, the communist USSR. Whatever brief mentions they had in G.I. Joe fiction eventually wound up showing we were in some Alliance against a "common threat".
If G.I. Joe had preached anti-Russian sentiment, I maybe would agree, but because it was all fictionalized I can't see any correlation to propoganda for the U.S.
Propoganda for better living... Moral behavior... Being "brave". That I can agree with. That's not "propoganda" in its truest form though. No more so than saying Mr. Rogers is propoganda anyway.
Being a good person isn't an "instiatution, cause, or a person" though, so I again just disagree there. To each their own though of course.
There’s no question that G.I. Joe is propaganda; while it may not preach a negative viewpoint, it preaches nonetheless, with its infamous tagline- “A Real American Hero” it states that heroism is built on militaristic principles and bravery weighed on how much damage you inflict on the enemy in the process of defending your home turf.
Again, I simply disagree... G.I. Joe's tagline of "A Real American Hero" is no different than if you applied it to a Fireman, or a Policeman, or a school teacher... It's just that kids react to military toys instinctively. It's not an intentional message that "military power to acheive victory is America at its finest" or anything similar.
Also, if anything, G.I. Joe preached pacifism at all costs... To not hurt your enemy but take them prisoner. The cartoon was so chock full of non-death it really does make you laugh now if you watch it.

You saved your enemy. Even the "real" enemies of the time didn't like who the enemy in the fictional world was and G.I. Joe/America and Russia even teamed together, working as one, to fight the fictional enemy.
So long as there's fiction there, which is all Joe is, was, and will be, I don't agree that there's propoganda there. If there is, it certainly didn't change me in the slightest, even if it was the most influential toy on my childhood.
To each their own though... It's just seeing things differently.