And yes, even C.S. Lewis was a sinner.
Any reasonable person would conclude that smoking is harmful and self inflicted harm is a sin. If it seems wrong it probably is.
And I recall Someone once saying, "He who is without sin may cast the first stone." I don't condone smoking, not even C.S. Lewis smoking. I suspect he even drank wine, or worse! But on the subject of one's eternal destiny, these are trivial matter, yes?