(copy and pasted from a comment I left in response to someone else)
I get people like to call this the "CalArts style" because shows like Adventure Time, Regular Show, Gravity Falls, and We Bare Bears are recent popular cartoons by CalArts alumni, but I feel it's also worth noting the vast majority of people at Pixar also went to that school, and so did Craig McCracken, the guy who first made Powerpuff Girls in the 90s (so if anything PPG was "CalArts" long before the reboot, also Lauren Faust went there too), Butch Hartman, Stephen Hillenburg, the creators of Rugrats and Ahh! Real Monsters, and hell even a bunch of Dreamworks directors and animators went there.
Plus the creators of Steven Universe and OK KO graduated from art school in New York, and Gumball is made in the UK, so I dunno to me it feels unfair to just single out this one particular school when frankly a lot of cartoons people love come from it's alumni, and I'm sure this art style is more rooted in something that goes beyond a single school when you've also got animators from New York and England using it.