Superman was supposed to fight by boxing rules, without his powers (though the aliens though he has them), and he stood his ground against the greatest boxer of all time, without any formal training. Imagine any strongman surviving something like this against the best boxer there is and you would be impressed.
On top of this, the fight was also very political in a political era. They explain how hard it was to make this comic happen with all the "race" wars during this time.
Been awhile since I read the comic (its 5 feet away from me right now, but don't feel like dragging it out), but wasn't Superman actually the janitor or somebody that dressed up like him? I seriously cant remember its been so long.
Ignore List - H1A8, Mr.Parker
Yeah, superman had trained with the knowledge that he wouldn't have his powers in the fight. If by "stood his ground" you mean that he got beat like a bongo while the woman that he loved shouted at the ref to end the fight and the fight itself ended in Ali walking away because Clark couldn't do anything other than block his punches with his face then yes, he did stand his ground. Also, superman isn't just some strong man. He has far more experience in do-or-die situations than Ali did and still got bodied so hard that his opponent quit after unleashing the sort of combo that you'd expect to see in a street fighter game.
Race doesn't have anything to do with the fact that Clark had to be carried away on a stretcher. Still was an awesome comic though.
Race wars had to do a lot with the concept of the comic. I have a newer re-release of it with an interview with the creators and they for example were not allowed to let Ali lose to a white man. An interesting read btw.