Smoking doesn't turn me on, but it doesn't turn me off either. Just like pornstars carrying a bag full of groceries doesn't turn me on or off. I don't mind seeing it in the intro or the outro of a scene/shoot though.
Of course, once they're entirely naked I prefer to see something else than a cigarette in their hands, depending on the kind of scene/shoot they do. This can be a dildo or vibrator in a solo or girl/girl shoot or a dick in a boy/girl scene/shoot.
So smoking is no problem for me before or after the actual action, but I don't want to see it during the action (but most pornstars don't smoke then anyway).
Also, a lot pornstars don't even smoke in real life as they obviously don't inhale (& if you don't you're wasting the cigarette).