The Art of Luca has 176 pages, with about 66 pages dedicated to 2d character designs which is great. The style of this film is probably one of the most distinct and different than any other Pixar film to date not counting some of their Pixar shorts.
To me it looks like the character designers were trying to go for this look and design rules that you see in many of the TV cartoon these days. Some people call it the "Calarts Style" even though I don't think it really started at Calarts. To me, it reminds me of a Wallace and Gromit smile. For an example of this almost 10 year trend in style all you need to do is look at some of the past cartoon shows from the last decade like Steven Universe, Gravity Falls, Gumball, Star vs the forces of evil, and the now infamous ThunderCats Roar.
Now we have Luca which is a 3D version of this same style. This style has been hotly debated over the internet on weather people like it or not and some hoping that the trend will end soon.. The design language mostly has to do with the overly simplified bean shape of the head and the simplified mouth pickle shape which always seems to be open, and makes the characters look like they are constantly worried and nervous, or overly happy. Like I said many people like this style and it reminds them of their childhood. I'm happy to see Pixar change it up a bit, play with different styles, and try to challenge themselves to push the boundaries of design.