I think Fluttershy is the most "appropiate" girl to wear diapers, but I have a headcanon about Pinkie too.
I think she discovered her like for diapers when accidentally diapered herself during the "Baby Cakes" episode.

And, after discovering their shared interest, Fluttershy and Pinkie have private slumber parties (just the two of them) where play videogames and board games, watch movies, eat pizza and candies, tickle until pee pee themselves and wear diapers…Please let me join, girls!


Traced poses from: https://twitter.com/letitmelo/status/1201842693384609799
by Melo.