A player who role-plays to his flaw may take actions that are less than optimal for himself or the party (and possibly even be rewarded with Inspiration for doing so) but even so, the rules don't force him to do that. If he does it occasionally to make the game more entertaining for the others, then that could be classed as "good". If he does it perversely, however, if he chooses a flaw that, in effect, says "I invariably make stupid decisions", to justify in his own mind actions that make the game less entertaining for the others (the "it's what my character would do" defence) then he's just being a jerk.
I would suggest that sub-optimal play is "good" if there is some legitimate rationale behind it, and it makes the game more fun for the other players.
On this issue my view is closer to that of BoldItalic than Maxperson.Whenever you roleplay your character appropriately, it adds to the game, even if it makes the goal more difficult. Good roleplay makes the game more enjoyable
You don't have to go all the way to CN/CE horror stories to see how "just playing my character" can reduce the fun of the table rather than make it more enjoyable, even if those at the table see the main goal of play being to establish and maintain characterisation.
I would push it one step further, though, and say that even if the player is not being a jerk, it might be a problem - at least at some tables - if s/he is not establishing goals for his/her PC and trying to move the game forward by declaring actions in pursuit of those goals. (And that doesn't mean that these games are hostile to "roleplaying". Though they may not regard characterisation as the be-all and end-all of RPing.)