Muncaster has made a good decision to branch out from Isadora Moon and make other characters the focus, as the Isadora stories could have become stale. What I like about these books is that Mirabelle (or Isadora) have real-kid problems that they can't completely magic their way out of. The solution involves some effort, either bravery or commitment. My 7 year old relates to this well, and because there is a bit of tension, its not completely boring for me as a parent to read either.
In this one, Mirabelle creates her own problems by being a bit sloppy/lazy. She leaves her broom outside, and it gets wet, so it won't fly properly, and she leaves her potions on the pillow and some spills in her hair.