That is not how economy works. Higher demand automatically means higher and more varied offers. It's not like the sweet potatoes will become luxury goods, because they are locally produced. By introducing them to the nobility she can create more interest in the growing and harvesting of more sweet potatoes and getting them to the market. Remember that nobles are the only landowners of medieval times, they are the ones who decide what to grow and what crops to plant. If they consider sweet potatoes vulgar and as something unlikely to turn in a good revenue because it's primarily consumed by commoners, fewer landowners would want to plant and harvest them, and fewer potatoes would make it to the market at large.
It worked though in the end it was mass produced for both noble and commoner consumption
i’m not trying to diss her or anything but she really saw people on the streets eating sweet potatoes and said ‘oh i’ll introduce this as elegant so all the rich people will want it’
which means the price will go up and they won’t be able to afford it anymore