I think Jango had his good points and his bad.  He was very human, which is the biggest complaint I've noticed when people say they dislike Jango.  I think he taught Boba the gist of being a bounty hunter, and the fact that he did make some mistakes could have given Boba a clue as to what to do and what not to do. 

Plus, it's always easier to see mistakes when watching someone as an outsider.  You don't know their weaknesses and faults when you're the one fighting them.  You don't have time to just observe, so when Boba watched Jango during his fights, I think that's where he learned the most.