Most people, sadly are close minded like that. No matter what you do to try to explain to them that Paganism is not evil they won't listen.
You can always try by saying "Mom, please sit down and let me explain. I never argued your religious preferences (If you haven't of course) so please let me explain a little bit about my religion with an open mind."
If that doesn't work then you can always research paganism and show her what it is exactly or go into the specific religion that you are. It's worth a shot.
She may or may not listen. It all depends on her. If she doesn't listen though, don't feel put down. It happens more often than you think.