JFFS2 is not designed for using it on regular block devices such as those implemented via USB storage or your hard drive controller - it's designed for embedded devices and when you have direct MTD access to the flash chips.
It isn't possible to use it on a block device except with some fairly advanced hacks, such as this one that details using a kernel trick called "block2mtd" to emulate an MTD interface on top of a block device. This is so impractical as to be an intellectual curiosity only, don't do it in the real world.
If you want a log structured file system on a USB drive, get one that's implemented in hardware - any Flash hard drive or USB stick which implements wear-leveling is essentially a log-structured file system at a low level upon which a regular file system is implemented at a higher level. While a mainstay of SSDs, this is still relatively uncommon for a USB stick with some exceptions, such as my "Sandisk Extreme" branded stick.
Or you may want to try experimenting with NILFS. But, it's much slower than traditional file systems at some tasks. You may be surprised to find good old ext4 to be faster than you thought.