…or even Muppets accepting the Christian faith. I am Christian, but I cannot condone Muppet characters doing something they wouldn’t do in a movie or any other media.
What about the Christmas films though? I understand why you shouldn't make it a major point in your story, though.
That said, I don't think there's much cause for worry about Mary Sues in this fandom, at least where the authors at this board are concerned
Although it does have a ton of self-inserts. And these self-inserts are often Mary Sues who join the Muppets, wow the Electric Mayhem with their gorgeous singing voice, become Janice's BFF, and end up dating one of the available members (Zoot seems to be the most popular, but there's probably only like 9 Electric Mayhem fangirls in existence). I actually created an OC, but I made her to specifically not be a self-insert. I ran her by my sister, and she said that my character was okay. I'm just saying, if your name is Rebecca, don't name your character Rebecca. And slightly changing it doesn't count.
Random Guy: SHE'S GOT A HETALIA OC NAMED HARLEY!
Me: IT'S SHORT FOR CHARLOTTE!!!!! AND NO ONE EVEN CALLS ME HARLEY! Yeah, but anyway, that OC is clumsy and awkward (not in the cute way, but in the clumsy and awkward way), and some characters even describe her as "weird".