You want to be a beta reader?! Lol. You are an illiterate imbecile, no way would I choose you. Get real.

Well this was definitely sweet and cute :3
And I can completely understand how the Mimikyus could feel, so lonely and sad. Being away from society can definitely affect them. Luckily Pikachu lost it's way and gave them the joy of Christmas :)
Well written. Your style is clear -cut and intuitive and unique, keep up the fantastic work
Machu _

You wouldn’t capitalize animal or mouse or dragon, so you shouldn’t capitalize words like pokemon or pikachu or charizard. The only time you should capitalize it is if you’re using it as the pokemon’s name, ie, Ash’s pikachu is called Pikachu. This is because you only capitalize when it’s a proper noun, which are the names of places or things. Similar reasoning should be applied to any other words you’re thinking of capitalizing, like telephone or trainer. Or professor.

[The Mimikyu pack led a mainly solitary life, their only company being the more curious Ghost types and the occasional Golbat.]

You know, there are other social structures than "pack".

[Margaret knew it was forbidden to depart the market without the leader's permission. And no one could recall the last time the leader gave permission to leave.]

For that matter, there are other types of "pack" besides "enormous group of individuals who all obey a single individual without question". It's not supposed to be functioning like a village.

[Margaret had closed her eyes, trying her hardest to visualize what it must've been like to have a person there.]

Why would a pokemon not think she's a person?

["Margaret, you are one of the strongest and most sensible of all the Mimikyu. Therefore, I am choosing you to investigate Christmas."]

Yeah, really not seeing what the extra layer of rules is doing.

Instead of there being some huge number of mimikyus (all of whom have somehow been living off one vending machine that's never been refilled but still has plenty of drinks) and then her getting ordered by one of them to finally leave, why not just have her be there and scared to leave? She meets the pikachu, she's curious about what he said, she ends up following after him when he leaves. Then she's actually making decisions and changing her own mind instead of just getting shoved around by fate.

[If the humans enjoy things like boxes and bells during Christmas, why couldn't her pack bring those things to them?

The Mimikyu would also bring the humans happiness, and be accepted into the city!

And when that happened, the pack would not be lonely anymore! They could be in the outside world forever!]

Similarly, it's a lot more believable that one lonely pokemon might think this, but really hard to swallow that she tells everyone and the leader thinks it's a brilliant idea, and the elders are fine with it, and nobody else has a single objection or memory of trying anything similar and it not working.