A common misconception I see that seems to get parroted is that Mary Sue means the female character in question...
Doesn't have a struggle
Is handed their powers/gifts/opportunity etc
Is more important than the other characters etc
While all of these CAN create a Mary Sue, the term Mary Sue comes from something far more insidious at it's core in my opinion...the death of ACTUAL femininity, AKA writing a female character more like a male character so as to "protect" females from negative stereotypes, all while creating NEW stereotypes that are equally or even more harmful in the grand scheme of things.
To illustrate this I will use a popular Disney Character who is both a great female lead...and a Mary Sue, yup it's a classic vs shitty remake
MULAN
In the animated original Mulan was a clumsy but relatable girl, she was in a world of extreme sexism and simply wanted to find out what she was meant for even has a song for such. He loyalty and love of her family caused her to take her fathers place to the call against the Huns despite knowing if she is found to be a female she will be put to death. Through hardship and brutal work not to mention using her intellect she is soon befriending soldiers and fighting alongside them despite rocky starts. This hard fighting Disney (Princess?) heroine is soon discovered to be female after making a risky tactical move and taking a blow that takes her out and is banished from the army as opposed to death as she had saved the generals life. In the end Mulan's friendship loyalty to China and skill are tested to take down the remaining Huns and their fearful leader in this animated classic.
In the new Mulan, she is the most skilled child with perfect Chi (chi is magical now btw) she is told the world (again extreme sexism) wants to hold her down and thus is told to hide her "incredible" talent. She doesn't join the army as it seems to be the ONLY way to save her father, but more as a convenient way to "prove" herself. She doesn't enter the army scared and confused forced to adapt and learn and thus overcome weakness like the animated version, but instead is better than ALL the other soldiers naturally despite her "chi" being weakened(Ill explain in a moment) The Huns are the Rouran army this round and have a witch (yup magical chi witch) who confronts Mulan and tells her that by concealing her womanhood she is weakening her "chi" Mulan CHOOSES to reveal herself to the general before being vanished and through the friendships that were thrown upon her and her natural talent defeats the witch (she was oppressed too FYI) and saves China...
The second just sounds droll doesn't it, the first film will be remembered more because the girl struggled, overcame and prospered, but she still was human and laughed, cried, failed and even needed help BTW see that flat face on the remake Mulan, well that's the entire movie which leads me to my point...
The thing is in an attempt to fix the weak female trope the entertainment industry has gone to far and all but removed the female aspect of most heroines, the result is an over powerful, often flat faced emotionless almost robotic character that doesn't NEED help and can do it on their own thus proving that they are worthy. The problem with this is it's the differences in our sexes that MAKE men and women unique, there is nothing wrong with overcoming that (again Mulan) but you don't need to sacrifice everything that makes women well...women in the process.
There will always be outliers, but women (more than men) can rely on sex appeal more than the opposite sex, they tend to be more emotional at time, they are built weaker when it comes to mass and muscle and are often smaller in stature and size. When we romanticize the human form they tend to be buxom and sexy and there is nothing wrong with that, nor is there a more slender build. The majority of women will find attraction to something they come across (most often a man) and relationships AREN'T poison nor is said attraction. Women can fuck up just as much as a man, and just LIKE a man those mistakes can HELP US GROW!
That in my opinion is the primary issue with the Mary Sue trope by getting rid of femininity we have robbed what could have been great characters of part of their humanity, and yes Aloy definitely fits into many of these latter issues, but she is far better than most.
Game still looks fire though lol