I send cards to people regardless of whether I get one in return. I just like sending Christmas cards! It's something my mother always did, and now I do it. People have been crossed off my list for various reasons over the years, but none of them because they didn't send ME a card.
But to answer your question, it's perfectly okay to stop sending cards to someone for any reason that suits you. Cards are optional in the first place.
Lots of interesting perspectives here. Shoo's is the one that most matches my own feelings. I don't include pictures, newsletters, or even personalized notes so my cards are very low effort. But, more effort than sending an email or posting "Happy Holidays!" on Facebook. To me they're just a happy little "I thought of you" to the person in question. And, honestly, I do it almost solely for my own benefit, because I enjoy it, and I don't notice who sends me one "in return" or mentions it to me. (Okay, actually, last year I noted who sent me one in return, but just because I like data and was curious. It did not in any way affect my list for this year and I don't even remember now who sent me one.)
If someone asked me to stop sending them or just generally mentioned their dislike of them, I would take them off my list, because I don't want to do something that upsets them. One person was removed from my list a couple years ago because they declined to provide me with their current mailing address when I asked for it (citing Christmas cards)--I took that as a sign that, for whatever reason, they didn't want one. No skin off my nose.
To answer the OP's question, I think if sending cards to certain people has become more burden than fun to you, you should just stop sending them to those people. Make your list of card-receivers a list of people you can smile at, not people who make you anxious re: cards.