Did I cause my mother’s death due to my negligence in preparing her food, and what is the expiation for my negligence and disobedience to her?

Her death was natural and not a result of error or negligence. You must be dutiful to your mother after her death by supplicating for her, maintaining ties with her relatives, fulfilling her will, honoring her friends, giving charity on her behalf, performing Hajj and Umrah for her, paying off her debts, and fulfilling her obligations to their rightful owners.