I appreciate that you said you would prefer not to talk about her susperstitious beliefs at all, but I don't think you really need much help saying a polite:
I'd rather not talk about that, thanks.
You do also ask if you should instead mention that you think it is "nonsense", so I would like to address what I believe is the most effective and polite way to do that. Also, you may need to fall back on this if she doesn't accept your polite decline.
I heard an analogy relating to the overturning of incorrect beliefs that I find really apt. Our beliefs underpin everything we know and trust, so they are like supporting structures, say for example a beam that supports a roof. If you want to replace that support you cannot just knock the old one - everything would come crashing down on you. You put the new support in first, then the old one becomes redundant.
Maybe you get the analogy already - but when you tell someone their beliefs are wrong, it is like knocking away that supporting structure. Everything comes crashing down on your head. Rather than attack someone else's beliefs, just affirmatively state your own beliefs. Give sound reasons for believing what you do, and examples that prove what you believe to be true. This is like putting in place a new, stronger support. If they find that what you have given them supports what they know to be true then they will accept it and their old belief naturally becomes redundant.
If you have a specific belief, tell her what it is. Or if it is simply that you don't believe in such superstitions, then just state your own reasons for thinking as you do and avoid criticising her directly for believing in them.
For example you could say something like:
I respect your right to believe that. Personally, I don't believe that anyone can be cursed with bad luck. The evidence I see in the world around me is that some good people are happy, but sometimes bad things happen to good people. Sometimes bad people get caught and have to face consequences, but other times they get away with it. Bad things happen to all kinds of people sometimes and there isn't any pattern to it. I find that belief comforting when I'm facing difficulties because I know that these things can happen to anyone and it isn't the universe picking on me.
Sorry that is a lot of my own words, and maybe not exactly what you would say, but my aim would be to:
Show respect for her right to believe what she does
Give reasoning for why you believe differently
Show how you benefit, mentally, from believing the way that you do