Should I defrag my SSD before creating a partition for Boot Camp?
The Boot Camp Assistant will move any file (if any) which may reside in your future Windows partition to a appropriate position in your then shrinked Mac partition before creating unallocated disk space in the repartitioning process.
Therefore there is absolutely no need to defrag in the forefront.
Please read Do partitions on SSDs map to physical addresses?

Or with other words:

A SSD works like an Amazon distribution hub:  

  

and a HD like Walmart:



If both of them want to sell a new product (let's say windows) Amazon just has to find empty boxes to stack all the different windows. The controller knows where they are. However Walmart has to free up a whole section to present all windows at the same place for the sake of a convenient shopping experience. ;-)