I'm thinking that most psychics choose not to smoke since nicotine can act as a depressant, which lowers neural transmissions and thus stifles their psychic ability. However, since Kira's Deviation is Pathos, meaning her psychic ability is influenced by her emotions, her telepathy becomes increasingly difficult to manage when she becomes stressed. She starts to read the minds of everyone in her vicinity, forming a cacophony of innermost pains that only she can hear, and at worst she could unwittingly hurt people nearby in a telepathic feedback loop or by unwittingly commanding them to harm her, themselves, or whatever else. She may choose to smoke as a coping mechanism against this, since the depressants in nicotine help to dull the stress and muzzle her psychic power until she can calm down. Plus, it would make sense for the time period.

Of course, she could choose not to smoke for a similar reason to her cooperatives. GUNS-BLAZING doesn't smoke, not because of the way it affects his psychic abilities, but also because he's a public-facing celebrity with an especial appeal to young viewers. He adopts a He-Man/Hulk Hogan-esque persona (he's a good actor, but in reality the "persona" is only about 2% different from his actual personality) and promotes healthy habits to his young viewers. "Winners Don't Use Drugs" and all that.