Normally, a healthcare proxy provides the ability to make medical decisions for someone who is no longer able to do so. Whether the hospice had an obligation to contact you with regard to your mother's care depends on when she became unable to make her medical decisions. My mother-in-law, who died of lung cancer, was lucid and making her own decisions up to within about three days of her death. If your mother became unable to make her own medical decisions and they failed to contact you about a course of treatment, then you might have an issue. In reality, there likely were no decisions to be made because all they would do at that point is continue to provide palliative care to keep her as comfortable as possible.
So, no, I don't think you have a legal case against the hospice. I would, however, contact the hospice administration and let them know that apparently their own internal procedures were not followed in this case. When we had hospice care for my mother-in-law, it seemed clear to me that a part of the care that is provided involves helping the family members deal with the upcoming death. In many ways, I think that's almost as important as caring for the patient. So, if they failed in that regard, they need to know about it.
I'm very sorry for your loss.