I do not want to start an “is smoking a sin?” debate.
Whichever side you fall on, most agree that it is not generally desireable to have any personal habit or activity that offends the people around you, whether it be smoking or biting your fingernails.
I would say that I think scrupulosity is a much more serious spiritual problem than smoking probably is.
To quit smoking is good. Whether it is sinful or not does not matter. Anything that gets bewtween you and God should be eliminated to the best of your ability. By anything I would include any activity, even supposedly good activity, that distances you from God, inhibits your prayer life, interferes with your duties assigned by God, like parenthood, or is stumbling blcok to performing corporal works of mercy.
Even too much prayer can be bad. Should parents spend 14 hours a day in the Adoration chapel and ignore therr children? Of course not.