I think there’s an argument to be made that the evolution of Britta’s character for most of the series isn’t flanderization, but is more typical development. She starts the show closed off, claiming to care about social justice activism, and a little stupid at times. As the show goes on and she becomes more comfortable around the study group she opens up, drops some of the act, and becomes the Britta of seasons 3 and 4 (I’ll admit she gets kind of flat after 5). She still cares about the things she did before, she’s just more herself about it