I thought of that as a possibility, though Rarity has been in a situation like this before and funnily enough was the first time we saw Manehattan (Aside from AJ’s flashback in Cutie Mark Chronicles). Suri Polomare plagiarized Rarity’s dresses. So Rarity had to suddenly find new fabric, and luckily enough she had the fabric in the hotel. 

That was a pretty desperate situation, and I think with how much it seemed Rarity was willing to help Pinkie she might of done something like that again. (I bet Pinkie could of had Rarity make it out of hair from her floofy mane and/or tail if need be. That might of been worth it just for the look on Rarity’s face. Pinkie possibly pointing out something like “Wool is just sheep hair. What’s the big deal, Rarity?”). A pouch would require less fabric then an entire line of clothing like with the hotel too. She could of probably made one with something as small as a towel.

But yeah it was probably mostly because the episode would of been over too early had they gone with that solution. These were new writers who wrote the episode, it’s something I can pass as something they overlooked.