When people over-react to something in this way, it's a sign that you have touched a nerve.
Your mum knows that smoking is bad for her, and for you, and probably would like to stop, but is unable to do so. It may be that it preys on her mind, so when you mention it, she feels a totally disporoprtionate sense of attack, and defends herself more vociferously than is necessary, and probably than she intends.
It is an over-simplistic answer on here to simply say 'if you don't like it, move out' without knowing your circumstances.
I think you should certainly leave the issue alone, unless your mum brings it up, in which case you can probably discuss her fears more realisitcally with her.
It's not an easy situation, and it will take a large amount of patience and tact to help your mum see that help is there, and all she needs to do is ask.