Depends how deep you're talking about. You should certainly understand how it works; and hence its strengths and weaknesses. But do you need to know the deepest darkest guts of its code? Not really.
To break down an example, take good ol' jQuery: you should know it uses Sizzle, creates its own element collections, understand how it traverses the DOM, etc. That level of knowledge is critical because it impacts the code you write on top of it. If you prefer the new and shiny version, you need to understand React's virtual DOM, etc...
But do you need to have read its source code line by line and understand every last trick and nuance before you use it? No. It's a great learning experience and you definitely shouldn't be afraid to dive in there and take a look, but it's not required to write good code.