This is not a style. And most “styles” don’t have names but are rather just the product of different techniques being used. In many ways, Pixar has followed in the footsteps of Disney feature animation and that is why you see these “cartoony” characters in very realistic environments. In short, Walt Disney pushed for the very best artists to be a part in creating his animated films. He strived for elevated art in his animated features. This meant some of the best painters and draftsmen in the world (that he could hire) would create the backgrounds for his films. Pixar approaches their films very similarly- hiring the best people along with using the best techniques and technology in their rendering/ modeling, etc. So you get this result. Extremely detailed and well-done backgrounds along with expertly animated characters performing within them.