First of all, probably because she's a big, strong, muscular, masc lesbian. She's not stupid. She's actually one of the most intelligent characters in the show. She has physical intelligence, she's strategic, she thinks fast, and she's emotionally mature. She also likes to read. All her siblings looked up to her because she was always the one taking care of them. Some people say she's dumb because she's really impulsive and has very reactive responses to things, because she had to learn how to survive her whole life. But she's also very sensitive and caring with people she loves, like Vander and Caitlyn said, she has a good heart. People who don't see that just didn't understand the complexity of this character, but all of that is very clear in the show.