How Nietzsche already remarked, when you donate to others you don't do it for pure 'good' reasons, you wish to be acknowledged as 'good' thus making it for yourself. In this sense Saitama is a heroic figure because he refuses to reduce his heroic status by the judgment of the other. Like in Camus' "The Stranger", the protagonist is a heroic figure which isn't understand by society and avoid normal societal normative judgments of behavior. He firmly stands for himself.
I believe One Punch Man is much more than witty commentaries and ironic mirrors of superhero trophes and motives: irony as a perspective through which watch our ideas of heroic identity and behavior. Why are we so obsessed by the hero and rhetorical myth of the savior? Which elements make the hero appealing and worth of praise?
If you want to dig deeper in one of the most repetitive and widespread archetypes of narration ever -- the superhero who saves us from the bad guy -- then One Punch Man is the anime for you. A dazzling picture of a normal man that doesn't have any real superpowers and yet is hero per antonomasia.
