Is it permissible for my grandmother to perform ablution (wudu') without wiping her ears, given the difficulty of removing and reinserting the hearing aids she wears?

The majority of scholars are of the view that wiping the ears during ablution (wudu') is a Sunnah. Therefore, there is no blame on your grandmother for not doing it, and her ablution is valid. However, it is better to observe this Sunnah.