The Last Of Us is a story that hinges on big themes.
The first game and first season were about love, the (crazy) lengths people go for the ones they love. The TV show was able to pull it off in an awesome way by retelling and adapting that theme by givings Joel & Ellie's journey plus secondary tales that were expertly done.
The second game and second season is about revenge. And this is when shyt gets tricky. Revenge is much more difficult to pull off, it's why the game is much longer and why it had more detractors than the first one. It took the player to places it was not comfortable to be in. But in the end, the majority of people were able to appreciate the journey ND took them.
The TV show has done a really poor job of developing the revenge theme while at the same time, going 1 for 1 with a ton of events, encounters and sequences from the game, so there's an obvious disconnect there. They trying to do the same thing while not taking as much care of the characters and their motivations. Ellie is the biggest example. In the game it's not only you as the player that wants her to start this revenge path, it's her whole close circle (Tommy, Dina and Jesse), and you have a really good understanding as to why she's going on this dumb ass suicide revenge mission (what Joel meant to her and to you as the player).
This season's biggest weakness was showing Ellie's motivation and making the audience understand her actions. The worst part of the show were the scene where she's nothing but a bumbling teenager that doesn't know what she is doing while putting her loved ones in huge danger (and getting rightfully scolded by them). TV show Ellie goes constantly changes from clueless to revenge driven and you never get to really hear or see what's really going on with her. The writing doesn't let her breathe and take in anything, she just goes from one extreme to the other because the plot has to move forward because we are on a budget and we have to wrap shyt up
If they decided to go with a more "realistic teenager" Ellie, then they also had to come up with much more solid writing around her than just making her go through the same sequences she does in the game. There's a lack of character exploration with her that hurts her as a MC.