Is it considered an act of disobedience for a son to refrain from fulfilling his mother's frequent and unnecessary financial requests, knowing her need for money, and while bearing other financial responsibilities towards his siblings and his marriage, especially since she uses the money for impermissible things like smoking, and refuses his advice to quit?

The status of the mother is great, and being dutiful to her is among the best acts of devotion. Allah has given specific injunctions regarding her in the Quran, and the Sunnah has allocated three-quarters of dutifulness to her. Among your greatest acts of dutifulness to her is to supplicate for her repentance from smoking and to advise her gently. You are not obliged to give money to your mother if she is not in need of it, and you will not be undutiful by not giving it, nor are you obliged to incur debt for that purpose. The majority of jurists hold that the father has no right to his son's money except what he needs without causing harm to the son. If the mother requests money without needing it, give her the amount that does not harm you, in order to gain her رضا (satisfaction), as this might help in her reform and acceptance of your advice. Strive to manage her affairs and gain her satisfaction, and seek help from advisors close to her if possible.