I've got an outlet box that is fed by #12 Romex backed by a 20-amp
breaker.

If I were to tap that box to feed a ceiling light, would #14 wire be
code-legal for box-to-ceiling-light?

If I were to pick nits, I might say "No" because conceivably somebody
might plug a load into the ceiling light that exceeded 15 amps....
OTOH.... ?
--
Pete Cresswell