HELLO THERE!
I want to make a little distinction between illustration and a fashion drawing. In illustration you can draw (almost a must) all details, colors, wrinkles, backgrounds, etc. of a character. This must be very impressive, dramatic and beautiful. But in a fashion drawings, lines must be clear, with minimun wrinkles, simple but very understanble. Why is this? Because the fashion drawings must be the guide for the clothes you are designing, this is the beggining in many cases, in some industries this is the first step to acept or dont the design. The next step would be the technical drawing of the garment, so the industry can follow