To me that means he thinks he's unbeatable …
Hmm … I don't get the impression that Zoro thinks he's unbeatable -- I get the impression that Zoro thinks that in order to become unbeatable, he must first convince himself (and then then others) that he's unbeatable. And how to do that easily? By belittling the enemy. How else can he quickly summon up enough nerve to fight enemies who are much stronger than he is?
Look at Zoro's body language when he first meets Mihawk -- his jaw is dropped, his eyes are practically popping out, and he's sweating like crazy. Then a few seconds later, he pulls himself together, gets a grip, summons up enough nerve and belittles Mihawk by making a formal challenge to him.
And that's probably why Mihawk does two contradictory things in response (he calls Zoro a "weakling", but also calls him "fierce") -- because Mihawk has been able to read Zoro's body language and see through bragging.
And to realize that it's both a sign of Zoro's weakness as well as a sign of Zoro's strength.
Hopefully this makes sense? I may not be explaining it very well.
Edit:
Well done Urokråke! Your post came through while I was composing mine. I'm in complete agreement with what you've said.