Is it permissible for my mother to stop reciting the Quran due to the difficulty of observing Tajweed rules, knowing that she becomes angry quickly when trying to guide her?

There is no harm in reciting the Qur’an without observing all the rules of Tajweed, because not all of them are obligatory, especially for someone unable to learn them. What is obligatory among the rules of Tajweed is that whose ترك (abandonment) leads to a change in the structure (of the word) or a corruption of the meaning. As for what is a "craft-based" obligation (for the perfection and mastery of recitation), one who leaves it off does not incur sin. One should be keen to honor their mother and advise her gently and kindly to continue reciting the Qur’an and learning its recitation.