If potatoes were grown in space would they taste the same and would Mcdonald's still flourish on the moon?
I doubt that the potatoes would taste exactly the same as Earth potatoes. Even potatoes grown in different regions of Earth taste differently. That has to do with soil composition (minerals and nutrients within the soil). I don't know what your space potatoes are using for soil, but if I had to guess, I would assume that some kind of artificial soil is being used. That is going to throw off some of the flavor profile that you are used to.
Would McDonald's flourish on the moon? That depends. How many people are living on the moon? If it is only a few dozen people, then no. But if we are talking about a thriving moon base with hundreds of thousands of people on it, then yes, I think McDonald's would flourish. It seems to flourish just fine in many towns across the United States that have much lower population numbers. Of course I do think an In'n'Out Burger would work better.