I love Day of the Triffids, such a classic - and honestly one of my favorite science fiction works ever!

In answer to your question, though, I think it's fair to say that many (if not most) adult readers would have encountered that kind of moral relativism prior to reading Wyndam's book. Early- and mid-twentieth century anthropologists were all over this idea:

Early on anthropologists accepted the assumption of European or Western superiority. But this was challenged by Franz Boas, and his students—in particular, Ruth Benedict, Melville J. Herskovits, and Margaret Mead—explicitly articulated influential forms of moral relativism in the twentieth century. In 1947, on the occasion of the United Nations debate about universal human rights, the American Anthropological Association issued a statement declaring that moral values are relative to cultures and that there is no way of showing that the values of one culture are better than those of another.

Even earlier, you had people like Westermarck insisting that moral judgements were never objective, and had no inherent value beyond the value that their context gives them, in The Origin and Development of Moral Ideas.

Both Westermarck and Anthropologists like Boaz and Mead arrived at the conclusion that morality was a "social construct" (although I'm not sure they would have used that phrase, exactly) in much the same way; by observing other cultures. Mead's study of Samoa, for example, and Westermarck's study of Morocco both led them to conclude these societies' (dramatically different) moral values weren't inferior, but simply different - and that the very existence of these different moral codes meant that morality itself was variable and relative.

The concept of moral relativism, then, definitely pre-dates Day of the Triffids. As to whether the "average" reader would have been familiar with the concept, that's a much harder question to answer but I'm inclined to think that an educated reader would have had a hard time not being aware of it. Whether they believed or scoffed at the idea, of course, is another matter entirely.