I think when they decided to spice Jadzia up, they were said to have decided to "make her Errol Flynn."
It makes sense that there would be a certain degree of... inconsistency in a person who had lived previous lives as other people, but as others have indicated a greater degree of overall structure to the character would have been great. The whole "she's an old man in a young girl's body" gimmick played out pretty quick, IMO, and they didn't quite know what to do with her.
I don't think Farrell is a bad actress, actually. The best ever? Nah. But not bad.