I'm really digging the material in the preview. It's got oodles of charm & personality, which reminds me that Pendleton Ward (of "Adventure Time" fame) spent some time up at the WotC campus -- not that I've read anything about him working on this specific project. Absolutely love the Pudding King (who would fit right in in the World of Ooo), the Kuo-Toa cleric who dreams up new deities each night, and the mind-flayer with the capped missing tentacle. They show an oddball, yet oddly appropriate, kind of attention to detail official D&D products could use way more of.
Regardless of whether you like whimsy or not, it's a traditional part of D&D, especially the Underdark. Insane fish people armed with metal pincers who worship something called "Blibdoolpoolp" are a perfect example of how "dark" and "whimsical" can go together like "gin" and "tonic". This is not fantasy in the grand, dignified, tradition of Tolkien. At beast, it's like Moorcock on a bender taking the piss out of Robert Howard.
What might be lost on contemporary audiences is how silly & surreal D&D sounded to young fantasy readers coming straight from the "Lord of Rings", C. S. Lewis, LeGuin's "Earthsea", and the "Chronicles of Prydain". It was a ghastly, goofy, floating eyeball & pudding & ooze-filled mess. Which was also irresistible.
The Underdark should be, well, kinda dark, but also surreal and more than slightly ludicrous/whimsical. It's, again, tradition! (and also like "Adventure Time", one of the best TV shows of the decade).