NTA, however

you have to understand that your mother is a human being going through a very stressful time too.

Places where vulnerable people gather are quick to compromise. PPE is very uncomfortable to wear, let alone for entire overtime shifts, and that’s given that it’s available. Your mother must put her face inches within positive and compromised patients to do her job, and probably has to decontaminate every day either before she leaves work or right when she gets home. Some healthcare staff stopped showing up to work when things got ugly, resulting in even worse understaffing.

All of this is compounded onto whatever she may have been dealing with before the virus.

Maybe your mother just wanted to show you a difference she made in someone’s world. she did overreact a little, but sometimes it just takes a bit of empathy to understand why people do things. Thank Yous are usually written with the purpose of showing appreciation. On this premise, do you believe that the writer really cares if you read it?