“Magical Mystery Cure” picked exactly the right ponies to get each cutie mark. Each one basically got their worst nightmare.
Rarity has to spend her life working on something that has no permanent results, a chaotic system to which she cannot bring order, and which pretty much always has to be done the same way–interesting weather is pretty much definitionally bad weather.
Applejack has to spend her life creating fru-fru things that are always different, no repetitive chores, and all of it done in the isolation necessary to art, away from her family.
Pinkie Pie is back on the farm, doing a never-ending stream of chores. It’s like a return to her much-hated childhood.
Fluttershy has to get up in front of crowds of people and expose herself to their judgment. If she fails, they dislike her, if she succeeds, they laugh at her; a perfect lose-lose for her.
Rainbow Dash has to stay in one spot, doing a job that requires gentleness and care, where the center of attention must always be on the animals, never her.