Very little. I think its popularity is almost exclusively due to his playing Superman in movies people enjoyed many years ago. Sure, people feel like they got to know him beyond those roles I guess and care about him (and his plight) as a human being. But that's not why folks want a Superman figure I don't reckon.
As for being a hero, sure, he was pursuing a cure to what ailed him, but forgetting that, he still focused his time and energy on something worthwhile--something he didn't have to do, and something that he must have realized wasn't going to bear fruit in his lifetime. The vast majority of people that do good work are doing it, in part, out of self-interest (researching cancer partly for money, becoming the chief of police partly for prestige, etc.). That doesn't take away from the fact that they are doing good work when good work is done. I don't think you have to be 100% altruistic to be a hero.