on mari, and mari's hair: it was so much more than just shauna having a last laugh, exercising her fist of power one last time over mari.
mari was a character who was meant to represent where true power lied, at almost all times. at the beginning, during the plane crash, she was always offering to help jackie, was always by jackie's side and agreeing with her, and as jackie's power slowly faded, so did mari... to lottie.
mari worshipped lottie, participating in the prayer circles, believing in the wilderness, and always advocating for lottie's power. she said lottie doesn't need a gun, because she truly believed that, she truly was on lottie's side (as an acolyte rather than a legit friend, but still).
she truly bowed to natalie, and followed her plan throughout season 3, because natalie had legit influence over everyone.
but shauna? she never caved to shauna once. she never believed in shauna, worshipped shauna, bowed to her. everything shauna had, influentially, over mari was strong armed over her. and shauna knew that. we see shauna stare out at jackie and mari in season 1, when mari is talking jackie's ear off on the beach rather than playing in the water with the rest of the girls. mari is the one who helps take care of lottie after shauna's beat her up, mari wins in the capture the flag game when natalie is queen, mari doesn't even succumb to shauna's influence during the vote on coach scott, because what shauna has isn't influence, it's power through force. and mari can tell the difference.
shauna taking the hair was her demanding mari worship her, demanding mari be stripped of her ability to chose and to be humanized because mari never truly worshipped shauna. mari was always the truest acolyte, not to the wilderness, but to influence. she was the truest teenage girl, trying to follow wherever popularity led to survival, as if in highschool. shauna taking mari's hair wasn't just cruelty, although it was cruel. it was shauna ensuring that mari had no choice but to be decorative to shauna, be a part of her queendom, be tethered to the queen she always hated.