Wishing for death is not permissible unless a Muslim is afraid of Fitnah (trial) in their religion. The Prophet (peace be upon him) said:
"None of you should wish for death because of a calamity which has befallen them. However, if it is (Part No. 2; Page No. 324) unavoidable to wish, let them say, "O Allah, keep me alive so long as life is better for me and let me die when death is better for me."
There are many Hadith to that effect. The action of your mother is not permissible, but perhaps she is excused owing to her ignorance.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.