@Radish8 actually wrote a comparison of the BBC books: Comparison of Book Club Picks
I haven’t read Kiki’s, so I can’t say personally, but my sense is it’s considerably harder than other stuff from the BBC. The Girl Who Leapt Through Time is definitely harder than Zenitendou, though how much I have a hard time saying since I read it first. I’d say if you’re getting through Zenitendou okay, you can probably handle TGWLTT, even if it is a step up in difficulty (keeping in mind the book club threads for it will help a lot, especially at the beginning, where it will be the hardest.)