A sweet little fic, and Rarity was perfectly in character throughout.

I like it! A few little grammar mistakes, but nothing too serious.

I think the dark tag would be appropriate for this.

A character being forced into a situation where they're completely powerless to stop another person from doing whatever they want to them, all the while horrified by everything happening to them and under threat of blackmail in which their social standing (and subsequently their entire career) would be ruined.

This is actually pretty horrifying.

“Oh… I don’t know, Rarity,” Fluttershy mumbled. “I don’t think my animal friends would like me being away from them so much… and uh… it is kind of expensive just doing this weekly.”

You got me wondering how this would have turned out had Fluttershy been there.

“But Rarity, you’ve already been through all our treatments,” Aloe said. “There isn’t really anything else.”

You could always go back to something you've already done before you know.

“Wait, don’t tell me,” Rarity said, her excitement building. “I want to be surprised.”

The spa twins probably should have gotten that in writing.

Aloe shut the door with a resounding thud as a confused Rarity finally started to realize she might be in trouble.

Your just getting that now?!

“D’aww,” Lotus gushed. “Aren’t you just a cutie?”

Yes she is.

Now, here she was, a grown mare of high standards reduced to wearing foal’s garments. Tears bloomed in her eyes.

Aww don't cry Rarity!

You're a beautiful foal.

Rarity pouted huffily, causing Lotus to giggle with delight. "Okay, silly filly, why don't we finish getting you dressed up?"

Yes please!

She had to be imagining the thick, fluffy diaper around her waist, and the silky, delicate socks snaking up her forelegs right now, and the plush, comfortable onesie she was wearing. Oh Celestia, is some part of me actually enjoying this?

That's it Rarity. Get nice and comfortable in your new ensemble.

"Oh, now don't be camera shy Rarity. I just want to remember your adorableness forever."

And who could blame her?

"I wanna pway dwess up," she decided.

Little Rarity really is just the cutest thing!

"My pretty little princess," she squealed.

When the mare is right, she's right. Rarity really is a perfect little princess!

Five more outfits were tried on, each as frou-frou as the last.

As they should be for the most stylish filly around!

Lotus also had a wide assortment of foalish jewelry that she let Rarity try on, and even treated her as if she were royalty.

Aww! I wanted to soak in the reign of Princess Rarity!

Oh well, there's still plenty of good content here.

Despite what Rarity tried to convince herself, she had fun. So much so that she was almost sad to be put back in a onesie. Almost.

Too bad. Rarity enjoying herself is the best part of the story.

"A sleepy wittle foal like you needs her nap time." Lotus rubbed her nose against Rarity's cheek. "Yes she does, yes she does." Rarity couldn't help but giggle.

Daww! The little princess is happy!

"I love you too, sweetheart." She slipped the pacifier in Rarity's lips, and the suckling began. Lotus tucked in the covers, turned out the lights, and shut the door behind her.

Rarity snuggled her sheep happily. The last thing she heard before passing out was her own crinkly diaper.

Aww! Sleep tight, little Rare Bear!

"I... don't think so. No offense darling. Like I said, I enjoyed some of it. But I think the whole experience gets tainted when I... um... do my business."

So what your saying Rarity, is while you can't stand the diapers, you'd be totally okay being a little sub otherwise?...

I'd be down for that, not going to lie.