Is it okay to dirty flag in Blitz/Bullet?


Different situations call for different etiquette, but overall, flagging is part of the game and even if one situation is "poor etiquette", they can't blame you too much because you are trying to win. Besides, both players agreed to the time control; them not using time management as well as you is part of the game.

There's nothing "dirty" about flagging, because the player who loses on time does so because of his own actions.
You don't control how much time is used on your opponent's clock. Only they do. If your opponent flags, it means that they used up all the time they are allowed.
Players like to blame their opponents when they lose on time, but really - it's their own fault.