Yes, Ben Affleck would have been better off directing the new Man of Steel than playing Batman in it, because Ben Affleck is more suited to be a director than he is to play Batman. Ben Affleck's roles in the past are not the type that would lend him to being a very popular Batman. He should stay behind the director's chair.
At first thought it seems weird for Ben Affleck to play Batman in the new Man of Steel but upon further thought, he fits right in. When one thinks of George Clooney or Val Kilmer Batman is not the first role that probably comes to mind however they played good roles. Christian Bale was also thought to now be a good choice however he is, arguably, the best Batman yet.