I saw TLJ and it SUCKS

If I'm recalling correctly, it's strongly implied that Rey learns to use the Jedi Mind Trick as the result of Kylo Ren's probing of her mind. It doesn't just come out of nowhere; she was stealing his moves. This interpretation is further informed by her character's background as a scavenger. She would be somebody adept at identifying and appropriating things of value and lived in an environment where being adaptable, quick witted and decisive would be key to survival. It makes sense for her character, which had already been strongly implied as having some inherent connection to the Force, to be able make a guess at what he was doing and boldly attempt to emulate it. Rey is also a far more determined and strong-willed person than Luke was, initially, so perhaps that also plays a part in how quickly she can learn magic that is entirely reliant on having faith in its ability to do the impossible.

Of course, it was also fan-service because people love the Jedi Mind Trick for some reason. That's the main reason it was there. But it was set up properly by the film and her character and not as out of left-field as people make it out to be.