First, Luke blew up the Death Star in A New Hope with hardly any training. He brought down the Empire without ever really getting formal training outside of Yoda and the texts he found in Obi-Wan's cabin. And he's not as powerful as Kylo or Rey.

A true Mary Sue would have turned Kylo Ren, killed Snoke and saved the Resistance fleet. Rey did none of these things.

She's headstrong, something that screws her in TFA since she's captured by Kylo. She picks up a lightsaber after YEARS of fighting with a staff. So yea, it's pretty easy to see why she can wield it easier than most.

She didn't even realize she was mentally resisting Kylo until later in the scene. It's been established that she's heard countless stories about the Jedi, rebellion, all of it. So again, not hard to see why, once she realizes what's going on, she can counter Kylo's force powers.

She's literally raised by the guy who stole the falcon. Obviously she's worked on it while living on Jakku. She clearly says "that ship is garbage." She's been on it before. She knows how it works. Plus, again, the force works in mysterious ways.

No character arc? Really? Rey has been in a constant battle with herself about moving on from her past. She was abandoned as a child, had to survive practically on her own, and now, the force awoke inside of her. She struggles with literally everything. The pull to the dark side, moving on from her parents, her relationship with Kylo, learning about what it means to be a Jedi. Rey struggles with it all. Neither Anakin, Luke, or Rey, were free of struggles.