Don't use a 16-gauge adapter on a 12-gauge cord.

No. The adapter becomes the weak link. A 12-gauge cord can handle up to 20 amps, but a 16-gauge adapter is only rated for about 13 amps. If you plug in something that draws more than that—like a space heater or a power tool—the adapter will overheat, melt, or catch fire before the cord or breaker does anything.

The gauge of the whole path matters, not just the thickest part. You’re essentially turning a heavy-duty cord into a light-duty one at the connection. If you need an adapter, use one that matches the cord’s rating (or better, don’t use adapters at all—get the right cord).

This is not the place to save a few bucks.