Parody, I think, works best in the short form. I remember reading a story years ago that parodied Stephen King's works, and thinking it was pretty funny. I'm not sure if I would have thought as much of it if it had been a parody of an entire book, but the writer captured the specifics of King's style well enough to make me chuckle.
Doon made me chuckle, too, but only toward the beginning of the book. The first time I encountered the puns of the names -- Boni Moroni for Bene Gesserit; Kumquat Haagendasz for Kwisatz Haderach -- were funny, but the more I saw them, the less amusing they were. In fact, once the humor of the puns fade, I was left having to judge the book on plot alone, and there it was so similar to Dune as to not stand out. Plus, replacing the political machinations with a cooking competition, and making the religion also food-based, was so silly as to be stupid. Plus, there were some references I straight-up didn't get, likely because this was published in 1984, and the references have become dated.
On the bright side, the book reminded me enough of the main plot of Dune as to make me feel like I had re-read it. In fact, reading Doon made me want to read Dune again, so even if I won't get around to it soon, it's back on the list, so I have to give the book some credit for that, at least.