I have to disagree with you about maddie being a "miserable, hateful little snot" and a "narcissist pile of trash." I honestly don't see that at all.
I see a little girl who is continuously put on a pedestal by her teacher and peers and is expected to win in a somewhat false reality. Who is clearly extremely hard on herself and a huge perfectionist and when she is not perfect, she doesn't know how to deal with it. And the adults in her life offer no emotional guidance on how to handle and cope with the intense negative feelings a perfectionist has when they aren't perfect.
At the end of this episode, after she got second, people kept saying "Oh my god Maddie lost, I can't believe Mackenzie beat maddie, maddie's in second, maddie is the loser." It was never just "Congrats Mackenzie", Maddie's failure was always tacked on to every praise. I can guarantee you that, as a perfectionist, she was already saying this to herself and when she couldn't escape all these comments from the others, she visibly became upset. And when her mother started to point out the fact that she was upset, in front of everyone in the room, she tried covering it up and told her mom to stop but Melissa kept insisting, only drawing more attention to Maddie when Maddie was trying to hide the fact that she was upset. And to be clear I don't think she was upset about losing, I think she was upset about people pointing out her loss over and over and over and the feeling that she had failed and disappointed everyone.
This would be a normal response for anyone. She was happy for her sister. She hugged her on the stage and she had a genuine smile on her face. She was smiling when they first got back to the hallway when people were congratulating Mackenzie. It wasn't until people started to continuously point out her failure that she became overwhelmed and upset.
Considering the fact that the days before this, she was once again in the spot light for her Sia video, probably hearing the controversial comments about the video, it doesn't surprise me that this is how she reacted. She carries such a huge amount of pressure around with her. I don't think many grown adults could handle that all perfectly, let alone a 12 year old.
Melissa is terribly flawed, I won't argue with you about that.