In my opinion, card counting is applicable to online and physical casinos.
As others have pointed out, card counting does not work in online casinos. The deck is shuffled after every round.
Edit: Unless you are referring to the "video stream" online blackjack where it's a physical deck of cards. That may be possible to card count on as long as they don't shuffle the deck(s) in a shoe too frequently. But even there I have strong doubts. These live stream gambling services aren't naive.
Live blackjack online might be work for the card counting but you have to face the consequences of that since the house are more strict online because they can easily trace those who are cheating the game and the result of this might be worst than what you think. As I’ve said, this is not a good strategy and you are risking yourself just to get the easy money that you want, beside you must be good in counting which is too hard to master.