Is the appearance of a deceased mother in a dream without clothes, and her refusal to take them, attributable to her negligence in prayer or an old debt?

The soul of a believer remains suspended by their debt until it is paid off. If your mother had a debt, then it is obligatory for the heirs to pay it off from her estate if it suffices. Otherwise, it is permissible for anyone to pay it off as a voluntary contribution, whether the debt is owed to Allah (such as Zakat, fasting, or Hajj) or to other people.