Is it necessary to inform one's mother about not having helped her financially, or is it sufficient to repent to Allah and seek her forgiveness?

You are not required to inform your mother of your shortcomings. It is sufficient to generally ask for her forgiveness and contentment, because her right to financial support lapses with the passage of time, and general seeking of absolution is sufficient for moral rights. Strive to honor her and treat her kindly to compensate for what you have missed, and seize the opportunity of her life to attain Allah's pleasure.