He was designed as one, after all. What makes him more Badass in some respects is that his player, Cory, is the GM's personal Butt Monkey. His entire world is dictated by a Killer GM who has his sights set on humiliating Cory as much as possible. The mere fact that Zoro survives this constant assault is impressive; that he's able to pull off moments of incredible badassery despite the world constantly working against him even more so.
Nami blows up a sea monster. The DM thought it was so awesome, she didn't even need to roll for damage.
Just when Nami and Usopp need help the most, Cory rolls a 100, meaning Zoro makes his navigation check.
Phil: Did you just knock every single enemy back down the slope?! ...With only six attack rolls?!