"Her book is mediocre."
I'm with you there. Unfortunately, I believe this is another example where one is supposed to answer with what you are supposed to have learned - and whatever you do, don't deviate! I'm learning to live with it on the grounds I'm getting the learning for free.
I think that Duo wants us to understand that faible also means mediocre. Naturally they will try to do that in the same lesson where they introduced the word.
F.W.I.W. of the four dictionaries that I use, only Larousse lists mediocre as a possible definition for faible. Larousse puts it fourth.
moyen is much more suitable and it has already been introduced to us. Faible is a very bad translation as far as I am concerned
As I posted earlier I think that Duo is trying to get us to understand that faible means weak in a figurative sense as well as physically.
Moyen means average. I suppose one could have quite a discussion about whether that means it is mediocre.