Still, medieval fashion didn't have to be functional. Knights had a habit of wearing shoes which had the ends tied to their legs by little ropes/chains so that they could be gloriously long and pointed. Corsets aren't necessary for having tight clothes either - I'm no fashionista, but you can have something stitched to be tight-fitting without a corset (there's a bit more care in getting into them is all.
It's certainly true that you can make form-fitting clothing with unforgiving fabric. Sewing in darts, using multiple shaped panels of fabric, and a simple lacing up the back or sides will give you something well-fitted and not wholly impractical that you could, say, attend dinner and dance in. Whether or not that was actually common or fashionable in the middle-ages is something different I imagine, but, it is possible.
