The heroes of the society are the peoples on which other people from society look up for inspiration. Nature of their mainly depends on only themselves. I completely disagree with the statement that Hero's are the only way to measure the character of society.
Here author generalizes that the people in the society idealizes the heroes and their behavior will be the same as heroes. Here there is the assumption that the society's all people will be having same thinking and principles as heroes which have chosen, there can be people with malicious thinking which can't be generalized in this set
Another big assumption which we are assuming that the character of the heroes of society will be same for every person to person character changes there can be different thinking and character can't be decided only based on some person of the society
It's true that society chooses heroes as their highest ideals since they are chosen by society and society will always choose the best of the character to represent themselves. So a character can be manipulated according to the change of people.
Furthermore, there is the assumption that the hero's thinking (character) will be the same for his lifetime. Humans' nature changes in accordance with various factors and there is no guaranty that heroes' characters will stay the same. Since we are judging society's character with the heroes so it'll be also sporadic character change as heroes character.
On the final note, we can say that judging some societies just on the basis of society's ideal is not the ideal thing to do there are various ways by which society's character can be measured.