Because it feels like it's being handed to her early on. Luke couldn't successfully use Telekinesis until his second movie and even then it was established to be pretty weak at the time, he and didn't pull off a mind trick until the third.
And until the third movie, all Rey really did apart from being a fair fighter (established in her intro scenes), was an almost unsuccessful mind trick and the lifting of rocks. The only thing she was handed was an affinity for the Force but that just puts her in line with Star Wars protagonists. We've got a metaphorical space Jesus, the son of space Jesus, and the granddaughter of Space Hitler. Yes, granted at the time we didn't
know that (and it's one of the worst decisions TROS made but my issues with that movie would make the two year angry at TLJ people blush) and one could argue that it was a panic decision (it probably was; but one could also argue there were hints enough throughout 7 and 8 between her initial vision and her immediate choice to ask the dark side for answers) but accepting it as true, then having Palpatine blood sort of makes a mind trick second nature considering the old raisin tricked an entire government to make him the dictator for life.
But strictly for the sake of argument and using only TFA, what we are shown and told in the movie seems totally feasible within the context of Star Wars, what we know of The Force (which is fuckall since it's constantly undefined vague magic mumbo jumbo), and protagonists of Star Wars. Luke Skywalker is an outlier only because back in '77 no one knew that his bloodline was actually that of the chosen savior of the Force, but Luke Skywalker's biggest struggle and defeat in A New Hope happens around him, not to him. His aunt and uncle die (thus actually giving him what he wants anyway), his mentor of two days, give or take, dies, and he doesn't even realize how close he came to death because Han Solo takes out Darth Vader so he can make a one in a million shot by using the Force to guide torpedoes into a shaft. Doesn't exactly seem like the kind of thing an impromptu training session with a targeting drone and a helmet covers. He manages to stumble into a rescue and get out relatively unscathed despite having no plan
for a rescue and basically outwits an entire battle station's worth of enemy soldiers (who are said to be amazing shots in the very same movie) including Darth Vader himself.
Anakin is an exceptional individual who, at age 9, built C-3P0 while living in a mud hut and being a slave, is the only human podracer because of his insane Force bug count, flies a starfighter into a battle and not only gets aboard the enemy command ship but blows it up and gets away, and despite the entire reservation of the Jedi
still manages to get accepted for training despite him being both too old and having a lingering attachment and fears which ultimately lead to his turn. We accept all of this because we already know he becomes Darth Vader and we like Darth Vader.
Rey was left on a desert planet by her parents and from a young age worked as a scavenger salvaging parts and repairing things, fixing up junked ships, and playing with old flight simulations. She's absolutely unremarkable other than her knack for survival but she's conflicted internally. She's the audience. She's
us when her eyes light up at hearing about Luke Skywalker, at meeting Han Solo. Much the same as people could relate to Luke as a teenager wanting a more exciting life. She's taught herself everything because she's had to, she didn't have a mother or an aunt and uncle and a secret Jedi watching out for her. She's used to being alone, used to fending for herself, used to not being handed things. This is why she's the foil for the other protagonist, Kylo Ren. She has no reason to believe she
is special until she is told she is by The Force itself via Anakin's lightsaber and by Maz Kanata, and then confirmed by Kylo Ren. And so this character, who has heard of Luke Skywalker, the Force, the stories, the legends, this person who has had to work for everything her whole life is being told that she has an incredible gift, her first reaction is to shun it; her second reaction is to try it. No one is more shocked than she is when it works.
Her third and final reaction is to accept it, which is when she finally overcomes the villain.
She doesn't have an Obi-Wan or a Yoda or a Luke or a Leia but the thing is, she doesn't know she would even need one. What she does have is someone knowledgeable about the Force telling her how to let it guide her - something that Luke Skywalker echoes to her and something that Obi-Wan told Luke. The most important part of Luke's 'training' wasn't the helmet shit, it was Obi-Wan flatout saying "A Jedi can feel the Force flowing through him. [...] it obeys your commands." That isn't all that dissimilar to what Maz tells Rey, and then we have confirmation that Rey's eyes were opened to
possibility thanks to Kylo even if neither of them were aware of anything other than that they
both are strong in the Force.
If Luke Skywalker can blow up the Death Star because Obi-Wan told him how to feel the Force, then Rey can mind trick a guard (after two failed attempts, mind) because Maz Kanata told her how to feel the Force.
And as we see in the next movie, someone who can use the Force doesn't
need a teacher to tap into it.
When Luke used the Force to get out of the wampa cave, did anyone think "Oh it's because he was trained in two minutes by Obi-Wan in the last movie!" or did they think "Well he can use the Force and that's something the Force can do!" It shouldn't be so unreasonable to believe that Rey can use the Force to trick a stormtrooper. And yet...for some reason...it is. And now with the terrible hindsight, there's even
less reason for it to be unreasonable.
If we want to be truly cynical about it, then the obvious answer is Rey used the mind trick on a stormtrooper because Obi-Wan did it in A New Hope and TFA is not at all shy about wearing its influences so deep on its sleeve that it might as well be a whole shirt.
Because aside from what I've said, Rey is the hero of the Sequel Trilogy. It's about her journey, whereas Baby Yoda is but a supporting character in his series, of a species we know nothing about other than every previously seen member has been immensely strong in the Force, and fifty years of background we know nothing about, which means I'm willing to give it a benefit of a doubt, at least for the time being.
We've seen dozens and dozens of humans of all ages being Force sensitive and the two biggest Force users that the entire film saga is built around are humans. We've only seen three Yodas and Yoda being strong in the Force wasn't out and out a genetic trait of his kind but rather the result of training for like 800 years. Somehow I doubt season 2 is going to reveal that Baby Yoda is the reincarnation of Yoda Prime or that prior to being on that planet with Nick Nolte that he was being trained in the Force by Luke Skywalker or some new Force user. The only reason people accept Baby Yoda doing crazy things with the Force as the equivalent of a human baby is because it looks like Yoda and we like Yoda.
People aren't willing to give the widow in episode 4 the benefit of the doubt in knowing how to shoot a blaster in a universe that is constantly in a cycle of war but they are totally fine with a little baby healing people and helping his dad fight a beast.
Also let's be totally real here, what do we even know about how strong Yaddle is in the Force considering she bounced out the Jedi to let Shaak Ti hang in the big chairs.
Making Luke flawed and human is one thing. But turning him into the mockery of everything the character once stood for is another.
I don't agree that Luke was turned into a mockery at all. If anything the Luke in TLJ was totally in line with the original vision of a sequel Luke albeit considerably toned down. I won't get into it because otherwise I would be here all night writing another essay about TLJ but suffice it to say in my mind the Luke in TLJ was fascinating and fantastic and one of the biggest reasons why it's the best Star Wars.