How are you handling capitalization on the keys/labels?
Custom fields have been added to #Ditto! Merged, deployed, etc. Try it out on ditto.pub.
Code: https://gitlab.com/soapbox-pub/ditto/-/merge_requests/591
Discussion
They are not keys, they're names. It's completely free-form and they have no semantic meaning. See: https://github.com/nostr-protocol/nips/pull/1593
To directly answer: the user can capitalize if they want it to appear capitalized. It's up to you.