"The soup was bad, because of too many cooks."
Translation:La supo estis malbona, pro tro da kuiristoj.
Hmm. I think you might be onto something there, though I also think it might need to be "multaj -da- kuiristoj"? Not sure if that's the difference, or if it just needs to be suggested. You can try to report it as an answer that should've been accepted, and/or maybe bring it up as a discussion on the "Duolingo for Esperanto Learners" Facebook group.