In the book, Joe thinks that Kerewin's sun-eater is grotesque because it appears to be an eerie contraption that operates without batteries or any electricity.
Joe finds Kerewin's sun-eater disconcerting. Kerewin's sun-eater is basically a contraption consisting of a mirror (to catch the rays of the sun), a crystal (to which is attached copper wires), and two magnets. During the day, the crystal oscillates, seemingly of its own volition. Although the text doesn't state what kind of crystal Kerewin uses in her gadget, some watch manufacturers today use quartz crystals as oscillators to keep accurate time in their watches. The quartz crystal has what is called piezoelectric qualities: it produces an electrical potential if mechanical stress is placed on it. For example, squeezing the crystal is a form of mechanical stress.
However, where Kerewin's contraption is concerned, it's not immediately apparent to Joe how the crystal is oscillating. He's a little uncomfortable with what he considers Kerewin's presumption; she believes that her prized gadget purrs "nicely along eating sunlight." Being Maori, Joe very likely knows about Maui, the legendary Maori hero who long ago managed to capture the sunlight by ensnaring it in a flax rope net and beating it with an enchanted jawbone. So, the phrase "eating sunlight" has a spiritual, otherworldly connotation to it which makes Joe uncomfortable.
Meanwhile, Kerewin informs Joe that she's made other sun-eaters and that one works only if it's been touched by human hands (and happy ones at that). Hearing all of this only makes Joe uncomfortable; to him, Kerewin's sun-eaters are grotesque contraptions. Perhaps he fears that Kerewin may be meddling with unknown supernatural forces she has no ability to control.