jQuery, for example provides a great deal of value and makes doing things that are normally a pain in the butt, a lot easier.
But... libraries can only go so far. You still need to write your own on-page script to make them act the way that you want, and to make use of the libraries doing their magic.
Having a good grasp of JavaScript makes your on-page code, including that which makes use of the libraries, that much better.
It's sort of like asking if you need to know
Java well -- after all, there are libraries!