TFA is a well directed movie with a bland story. TLJ is an uneven story very unevenly directed - some parts great, some parts awful with a bizarre structure and pacing. TroS is 2 movies awkwardly tincanned into one and cinematography-wise, is about the best what could be expected from such a horrible premise.
By contrast, the prequels are all badly directed movies with awful and uninspired cinematography. Everything is flat and unengaging, characters look like they are 'floating' in some sterile universe with no sweat, dirt or blood.
Any way, to get back into topic, overpowered characters are not necessarily bad for the story. Lots of the most famous and successful fictional characters are incredibly overpowered in their own universe. I didn't mind Kylo and Rey finding and using new Force powers. In fact one of my peeves of the Prequel Trilogy was that the Jedi were distinctly underpowered. Lucas came up with essentially nothing new for their abilities compared to very basic stuff Luke was doing in OT. They were just using same stuff and wearing same clothes (!) as Ben & Luke in OT.
However the power should have purpose for the story. Some of the stuff Rey does has little purpose except "this is cool". It is not foreshadowed or explained in any way. For some reason, JJ & Kasdan wanted to keep it a surprise that Rey was a force user. Whereas her aptitude on mechanics and close combat is easily explained by her surroundings, and her piloting ability can be somewhat explained too, her Force power really comes out of nowhere and it hurts the story because it looks like she was already reasonably complete character and was given Force ability just for the cool of it. Although I do not think Rey is a Mary-Sue compared to some of the very obvious examples*, it does make her somewhat Sue-ish especially when combined with how powerful she and Kylo are said to be compared to other estabilished Force users (again, for no further explanation).
Does it explain why the sequels were badly received? Not really. I think Rey's character is perfectly fine - though poorly set up - and I rather like Ridley in the role. The biggest problem of the trilogy is that there is not really a coherent story, just bunch of excuses to come up with cool scenes. And here the finger primarily points at the producer who approved this approach.
*examples of much more obvious Mary/Canon Sues:
-Ayla of the Earth's Children
-Jack Ryan of Clancyverse
-Tauriel from Hobbit movies