I think when the ghostwriters picked up the series, it definitely took a dip in quality that returned on occasion when there was a well-plotted book, but I don't think you can blame it exclusively on the ghostwriters. I mean, TV shows are always written by writing teams, yes there is usually a show runner or EP who has executive story telling power, but other people write the shows. The emergence of the ghostwriters was more of a shift in concentration and effort I think than simply the introduction of new writers into the mix.
I mean, she introduced the ghostwriters because she started working on Everworld, right? I'm not like *grossly intimately* familiar with their career or anything, but it sounded to me like they were working exclusively on Animorphs up until they started writing Everworld, at which point they had to delegate. A perfectly reasonable move. And exactly how much weight they had vs. the ghostwriters is irrelevant, I think. There were some really good ghostwritten books. There were also some not-so-good ghostwritten books. KA came up with the plot, right? For each one? And she was busy with other things. And sometimes, it showed.
idk. The first ghostwritten book wasn't even that bad, was it? Wasn't it #25? And #23 was written by her, and it certainly was chock-full of filler, really the only reason that book is memorable because Tobias finally found out Elfangor was his father, and that took all of two pages. The last book KA wrote before the finale was the one where Rachel got split in half and I remember thinking that was the stupidest idea ever.
Anyway, in conclusion, I think it was a combination of creative stagnation, an exhausted premise, shifted focus, and yeah, some ghostwriters who maybe weren't as familiar with the characters and universe as they should have been.