How do you deal with it if it’s your mother that is the narcissist and she’s fooling everyone?
I would work on letting go of what others think. Many of us focus so much on defending ourselves to others when we are dealing with a narcissist in their smear campaigns. Instead, turn the focus on yourself and what you can change so that you are dealing with the toxic behaviors. This can include limited contact with strong boundaries and consequences for breaking your boundaries.
I’ve seen many that the abuse or toxicity of a parent had them go no contact until the parent (if at all) was ready to respect their boundaries and change their behaviors. The people she fools either do not have a strong relationship with you, to begin with, and will probably never believe you or they are healthy and just being manipulated and will come around to seeing the true especially as they see you distance yourself from the BS.