I’ve heard the japanese graded readers series mentioned a few times on these forums and have 1/3 of the level 0 books. Is there a resource that serves as a reading comprehension quiz for these books? Thank you.
You could ask people to ask you questions about the readers. 
Join the Graded Readers and Parallel Texts “Book Club” - we’ll help you out with any questions you might have!
Is there anything special I have to do to join?
No 
! Just click on the link and ask your questions in that particular thread - many of the members there will have already read the readers you’ve got and will be happy to help out.
And if you want you can add yourself to the list, show which of the books you have =)