My unconscious judgement is that the smoker is someone who does this all the time (which might not be true) and the fireplace-user is just lighting a fireplace

My unconscious judgement is that the smoker is someone who does this all the time (which might not be true) and the fireplace-user is just lighting a fireplace as a special occasion (also not necessarily true). So if I see a scene of a parent smoking, I will judge them as a bad parent, but not really the fireplace user. To answer your actual question, yes, doing something that is worse than smoking near your kids is bad parenting (unless it's a rare enough event to cause negligible harm).