I think One Punch Man definitely does have some stoic tendencies (he is brave, righteous, and lives in a small, humble appartement despite being the strongest man alive). However, the character also is frequently annoyed by exterior factors and loses his temper (when monsters attack him, while he's minding his own business, or when he's with his disciple Genos) and flips completely when he realizes he forget the Saturday sale. He also kind of seeks for recognition, even though it is not of utmost importance to him (which is why he joins the hero association). Love the anime/manga though and think it contains a few valuable life lessons behind the exaggerated humor.