She’s too perfect in that she not only has acquired more skills than ANH’s Luke in less than 20 years or so and in poor economic conditions, but she learns all she needs to know about the Force exactly at the right time.
It seems to me it is a lazy criticism - not understanding the characters in the SW universe, or not thinking through the events in the film itself as a whole (her environment, how she struggles initially to the events occurring around her, the decisions she makes, then how she evolves and adapts through those situations she encounters etc - ultimately ending with her ‘Letting The Force In’, as it were, to not be defeated by Ren in the lightsabre fight at the end of the film).
And here’s the thing: it’s not about adaptating to the environment, because it’s been enstablished with 6 movies that nobody in the SW universe can become a skilled Jedi without training, patience, diligence. Rey, on the other hand… Now, I know people’ve speculated that there might be a reason for her sudden knowledge of the Force (like some sort of training at young age), but it wasn’t even hinted or questioned by anyone in this movie, so even if this were true for the sequels, saying Rey is a deus-ex-machina or “Mary Sue” in TFA it’s still understandable. After all, even with a shared plot, every movie should be able to technically stand on its own.