I don't entirely agree. Rey is a poorly written character by any stretch of imagination. Like most of the characters in TFA tbh. We don't know her motivations, or goals, or desires. She just acts as the plot tells her to. She's tough, because she's supposed to be "empowered" and she's good at everything she does with little to no explanation. And look Kirito in SAO has the same exact problems, this is why people call him a Gary Stu.
Compare this to Mikasa. We know her motivations, and desires. While they might not be particularly deep or complex (I do agree she could be much better written), she at least has something to drive her, and a well defined purpose. "To become stronger to protect the man she loves." And while her borderline obsessive attraction for Eren is a bit much, it's not without explanation, and more importantly her natural talent suits her cool and collected nature, and serves as a contrast to Eren the MC, and show him as more of an underdog, and give him someone to measure up to.
Rey's natural talent is not only unexplained, but really, serves no purpose in the context of her story, and more than that it actually HINDERS the story, as it removes all dramatic tension. Against the odds at which Rey measures herself up against, she seems so overwhelmingly powerful that nothing is really a threat to her. She rescued herself, and even defeated the main villain of the story with zero training.
Mikasa by contrast is still arguably weaker and less skilled than Levi, and both a extremely vulnerable to the threat that are the Titans, as the show goes out of its way to show that ANYONE can die if they slip up.
As far a sexuality goes. Yeah, I agree that male characters don't have to be overly macho, but traits like heroism and courage are inherently masculine, they are testosterone driven, and strongly associated with masculinity. So by default, when having a female character that takes on a traditionally male role of a hero, she's already taking on various masculine traits. It's always good to balance these with at least a slight flair of feminine traits in order to capitalize on the fact that you have a female character, and more importantly, make them relatable. Even the most tomboy of tomboys, will still have a slight bit of a girly side to her. Because everyone in a way kinda carries masculine and feminine traits regardless of gender. This is why the new male lead in KR works so well, because by adding a tinge of feminine traits to him, despite him taking on a very masculine role, you instantly make him more interesting. This is why Jojo characters are so interesting as well. But despite the borderline homoeroticism in Jojo, all the characters are still very masculine and badass. Back to Mikasa, she still has some feminine traits. If anything, in the way she gets all deredere for Eren. It's that small detail that instantly balances out here character, and makes her that much more interesting.
Rey on the other hand is completely devoid of femininity and sexuality, and thus, she's incredibly boring. You could literally replace her with a man, and there would be NO difference. So then... what's the point?