I don't see why you need to use something as large as 12/2 to wire a single lamp.

OP is using 12/2 as this is being tapped off a 20 amp circuit!

OP is using 12/2 as this is being tapped off a 20 amp circuit!

Can one use 14/2 and call it a fixture whip?

nope, it is part of the branch circuit wiring until it hits that fixture. The exceptions for fixture wiring are more for the cords of pendant lights, fixture wire to a chandelier, or wiring within a sconce. I once ran across an electrician that believed the fixture wiring applied to wire feeding a track light and chose to run 18 gauge on a 20 amp circuit for the whips to the tracks! That is definitely the worse place to interpret that rule wrong, especially since the track could be loaded up with 20 amps of lights potentially. Although in the case of the OP, one little light using about 20 watts, 12/2 is still needed.

why not just plug the light in and have a switch on the fixture?