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"They ruined Britta" by what, fleshing her out? I'm so sick of this take being used anytime Britta is shown as flawed. She's an activist because she's super opinionated and she stands up for what she believes in, and that's her strength as well as her biggest flaw because she also gets caught up in her own perception of the world and fails to factor in other people's thoughts and needs (like when she paid the deaf girl to spoil Abed his book). She gets caught up in her jokes, gets caught up in her need to "treat" her friends, gets caught up in proving she's right. And sometimes she's right and she succeeds, sometimes she takes it too far and hurts people. It's her coping mechanism after having had to run away and to prove herself to everyone. She wanted, no, needed, people to see her succeed so badly, she sometimes fell into self-centeredness, even when her intentions are never malign.
So no, she didn't necessarily get ruined, it always made sense for her arc and she's a much more relatable character that way.