I think we should compare General Leia with Rey. And for fun and games mesa want to add Jar Jar to the mix.
General Leia
Unlike Luke or Han, Leia is a rebel leader from the outset. So while Leia’s successes and failures may not be as immediately obvious as Luke or Han, in many ways the rebellion’s victories and losses strategically rest of Leia’s shoulders.
Leia first significant scene sees her arrested by Vader. Instantly we see one person standing up for a righteous cause and now being punished and another choking the captain and reprimanding the underdog. In this scene we have established an emotional connection to Leia’s plight-save the rebellion- and grown a dislike for Vader. Fast forward and we see her resisting torture, another reason for the audience to like her. Then her planet blows up.
Once she is “rescued” Leia proves herself by going down the garbage shoot. But does this make her a Mary Sue? No. We have seen Leia suffer and now like that she is finally able to take action and eventually defeat the bad guys. And yet even then it is still a team effort- Leia’s plan is fine until the garbage begin to compress and now its up to R2 to save the day. This is called “teamwork” and it helps create dynamic groups where everyone gets their moment to both shine and fail- so that they still feel real. The Yavin battle is just as emotionally engaging from Leia’s POV.
I’m not going to do the rest of the trilogy because it’s more or less the same (albeit less a focus on Leia in favor of Han beyond 4). From what is implied between OT and ST Leia succeeds a bit and fails a lot, but she is on the right path during 7 and 8. We don’t see much of her though. I don’t have a problem with “Leia Poppins” from a canon POV, although I do dislike how it deflates the storytelling tension.
Overall, we can conclude Leia is a great character. Not a great female character. A great character. I don’t hear many people calling Leia a Mary Sue.
Jar Jar Binks
Now let’s take a look at a poorly written character. Jar Jar. From the moment we see Jar Jar we dislike him. He annoys our heroes. Qui-Gon then asks him to show them to Gungan City, but it shouldn’t have been that hard to find. He screams his way through the planet core. Messes with the pod racer and pit droid. Steps in poo doo. Keeps annoying everyone.
What can we conclude about this character? Well, he fails a lot. And he’s annoying. So we don’t like him. The fail : win ratio is off balance. By the end of the story Jar Jar is promoted to general because the script said so. Then he continues to fail in the battle and is saved at the last second by Anakin destroying the ship. Had TPM given Jar Jar some likable aspects- maybe his people had to flee their homeland and go into the ocean because of the droids, we would have a motivation from which Jar Jar can help out the Jedi and eventually free his people from the droids. But he never gets a chance to prove himself.
Rey
At first glance Rey is a likable character. She is in a tough situation but working through it. Unkar Plutt uses her by lowering the profits. Then she meets BB8 and she wants him to leave, but by the next scene she doesn’t want to sell him? It’d be stronger if we saw this arc on the way to Nima Outpost but ok. So far Rey is well-written with lots of potential.
Rey successfully escapes the First Order, but presses the wrong button and lets the squids out. Again this is a good win/loss ratio. They establish a weakness- fear of letting go of her past. It’s good. Then Rey beats Kylo Ren.
Prior to the TLJ, I didn’t think Rey was a Mary Sue. I thought the mystery set up would explain why Rey suddenly got her powers, and that next time Rey faced Kylo it wouldn’t go so good for her- Kylo wouldn’t have been wounded anymore. Instead, we got a film more interested in Luke than Rey.
I don’t have a problem with the concept Luke turning emo. The problem comes when so much screentime is focused on Luke, Canto Bight, and Poe’s mutiny; all of which ultimately lead to what would have happened had they not been there in the first place (Luke’s return, Finn and Rose doing nothing, the transports launching anyway). These distract from Rey, who does very little in TLJ. She is sad that her parents are nobodies, but didn’t we already address this in TFA? If not, shouldn’t there have been a couple scenes to set this up, maybe a conversation with Luke about his daddy issues?
When Rey and Kylo face off for a round two of TLJ, Rey wins off-screen. We have one character effortlessly beating the other twice, and another desperately seeking what they consider to be the right path but faced with great struggle. That’s Luke and Vader, only swapped. By the end of TLJ, Rey is projected to win despite having no clear motivation anymore (besides good guys are good) and a lopsided ratio favoring wins (the opposite of Jar Jar). Kylo on the other hand has a valid motive, several victories (Han, Snoke) and several losses (Rey, Luke).
So yeah, that’s why I’m routing for Kylo Ren in Episode 9. Had they been gender swapped same would apply.