I know it isn't a limerick but I was requested to do a "dirty limerick about rarity" so the "limerick" part stuck.

Rarity's Ditry Limerick

To Rarity, none's worse than dirt

To think of it makes her hurt

It stains her clothes, gets in her hair

It is something she can... not... bare!

But even so

With all her woe

She still finds time to preen

To braid her hair

Give it flair

Impart upon it: sheen

But in her pampering

You'll see her sampling

The finest of mud masks

They come from lands

Where mud is grand

To find out why, my task