Is it considered cheating if one plays Pokémon GO while being a passenger in someone’s car?

No, it's not cheating.

For comparison, you might like to know about the ingress speed limit (What are the exact speed limits for charging and hacking/deploying/xmp firing?). If you're playing ingress in a car moving faster than about 35mph, your app will stop working. This does not happen in Pokemon Go; therefore Niantic is okay with you doing it.

However you probably won't be able to get a cell signal (either GPS or data) while underground in a subway.

If you travel slower than 12ish mph, the game might think you were walking and hatch your eggs. That might be considered cheating if you exploit it deliberately.

It's very effective to sit in one place under some lured pokestops and just collect Pokemon without moving at all. I'm not sure if car-pokemon is any more efficient than that.