In a fabric store, I think it's not necessarily rude for a cashier/employee to ask what you're going to make. They usually sew & craft themselves, and they like to hear about people's projects. I often get asked what I'm making, and when I almost invariably say "a quilt", the workers usually have questions. I bought a variety of ribbons last weekend at a fabric store, and they asked what it was for, and when I said my daughter's t-shirt themed birthday party, they were riveted.
BUT! Dismissing your friend's project was completely rude and wrong. JoAnn's makes its profits off people who sew; it's not supposed to want us to all go buy premade stuff. As for the slam about the baby, that is unforgivable.
Also, it was completely wrong not to abide by the line procedure.
I had a similar, epically rude cashier at a pet supply store who was waiting on people before me and being rude ot me, and I complained to management. Sadly they did not fire the man (and he truly, truly deserved firing... a complete stranger said to me in the store, "Wow, I can't believe how rude he is being to you"), but the owner contacted me and told me that they had a consultant come in and study the set-up, and they moved some things and made changes so that the line procedures would work better. It is the right thing to do, to complain.