Fashion designer?

if you want to be a fashion designer do you need to be able to draw really well?

not necessarily
I used to take a fashion course ages ago, I quit half way but this is what I learnt;
you have to know the outline that a fashion designer has to draw her clothes on
you should draw that pretty well so you have a base to draw clothes on to
and when drawing the clothes, you dont need to draw perfectly but it has to be recognizable as to what kind of clothing it is
also, you shouldnt worry too much about the drawing because you have to lable certain parts
for example, if you design a dress
you must label the neckline and add a caption giving details about it.
and you must label and add a caption for the material and how its going to be sewn on, if there is going to be any layers in the dress etc
so in answer to your question, no you dont have to draw really well, it just has to be clear what it is you are drawing.
and you must always include details about each section of the design.