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.

While Niantic doesn't specifically state if this is okay or not (that I know of). They have mentioned cheating in their Trainer Guidelines.

They say:

Methods of cheating […] include at a minimum the following: using modified or unofficial software; playing with multiple accounts (one account per player, please); sharing accounts; using tools or techniques to alter or falsify your location; or selling/trading accounts.

Also the game still works when driving at speed so it can be assumed that it is intended that this is okay. Considering that there is a speed lock on hatching eggs. If they intended that you couldn't play while driving (as passenger) then they would have put this same speed lock on catching Pokemon as well.

Also note that recently the scanning speed was reduced from 5 seconds to 10 seconds. Which means that at about 60kmph you will be covering half the area you would've previously. This means you're going to be finding much less Pokemon while travelling fast as the scanner will miss some.