I can understand your frustration with your mother to some extent, but at the end of the day, she is an adult and it's entirely up to her how she chooses to live her life! Unless, of course, she suffers from dementia, and has been deemed as lacking the mental capacity to make decisions, which is a whole different scenario! Only in these circumstances, might you be able to impose your will on her as to whether she chooses to lock herself away, not socialising with others or whether she chooses to try and live her life as normally as she possibly can, in the current circumstances!
I can also fully understand why you might view her actions as selfish. However, this type of attitude isn't really that uncommon in elderly people of your mother's age! Many feel that they have spent the majority of their lives putting other people's needs before their own and now they just want some "me time" in the limited time they do have left!
There is also the possibility that if your mother has several health conditions which make life difficult for her, she really doesn't care if she contracts Covid-19 and actually dies from it! ? My mum's brother, who is 90, has prostate cancer (since 2004) and who lost his wife to dementia a few years ago, has refused to have the Covid vaccine! He lives in a care home and he is the only resident to refuse the vaccine! He doesn't have dementia, and has full mental capacity, so he is entirely within his rights to refuse to be vaccinated. He has been pressurised by the home manager and by my 2 cousins, who think he is being selfish! However, the reason he has given for his refusal is that as far as he is concerned, his life is over and he just wants to die! ?
I understand perfectly where he is coming from, but then I am a retired Senior Nurse with a career spanning more than 30 years. In my working life, I had to deal with people from all walks of life, whose views and values were often radically different to my own! I won't pretend it was easy, but I always tried to do my best to view things from several perspectives, rather than just my own! I couldn't have done my job otherwise!
Maybe, if you can, try to step back from the situation you are in, and imagine how you might feel if you were in your mother's shoes? I wish you the best of luck! ?