From what I heard, Nisemonogatari wasn't meant to be really 'deep' anyway. While most other stories in the franchise are supposedly more serious, Nise is said to have been written with no real thought behind it and while it has obvious connection with its prequel, there isn't really much new we learn, apart from Tsukihi not being who she appears to be. But, Koyomi doesn't care, so in the end, we're back to where we started.
Still, it's amazing that the show is doing as well as it is. If this were any other show, it probably wouldn't sell as much, or at all.