It is a bit strange having the fairly realistic animals and scenery but the cartoony guests.
However, I agree with XT and others that it saves processing power. The game tends to start bogging down for many players once the guest count rises to a certain level anyway, and imagine what it might be like if the guests were rendered in as much detail as the animals.
There is also an "uncanny valley" thing that can happen when human figures surpass cartoonishness levels of realism and detail but still fall short of full realism. You end up with animations that look kind of creepy and zombie-ish (maybe this is why GGG21 feels the Jurassic World guests look robot zombies with inflexible rods up their, ah, tushes).
For folks who haven't heard the term "uncanny valley" it was coined by a robotics professor in Tokyo and refers to a sort of creepiness many people feel when something approaches anatomically correct human-ness but isn't quite right. It especially seems to revolve around human faces that look realistic but are also just wrong in some way. It's why many people find human like robots, clowns, lifelike dolls, mannequins, and CGI graphics that are trying for realism but not quite there to be off putting.
I imagine it is a challenge for game designers, as there is a zone, somewhere between cartoonish and complete realism that turns many folks off.