Many times after making inferences about a person, I was admonished not to "judge" by some well-meaning figure of authority who would often follow up on this with "you don't know what they're going through." A variation of this is "don't judge a book by its cover." "You can't judge me" was a typical celebrity line.
Strangely, I can't even recall seeing discussion of this attitude on the internet. I'm not sure why since it was extremely prevalent not too long in the past, but if I had to guess, it's because it was a saying from the "colorblind" times. Obama and his ilk kicked off a new era in racial politics where it became permissible to judge white people for everything they did and said, so even a white lib can't really tell their kids "don't judge" when they are teaching them to constantly judge themselves for white attitudes like speaking clearly.
The notion of not judging comes from Jesus' words in Matthew 7, "Judge not, lest you should be judged." These words are taken at face value and ripped off in simplistic and tendentious ways, but the Bible is meant to be read as a whole, not quote mined, and the full meaning is closer to "don't judge superficially." GotQuestions has a great article on the topic. Indeed, Jesus also said, "Stop judging by mere appearances, but judge correctly."
So the Bible does indeed tell us that snap judgments are wrong. But it also calls on us to investigate, discern, and make more accurate judgments. Without the ability to do this human civilization, and worship of God, would cease to exist.
I'm not sure how many parents are still repeating "don't judge" to their kids, but instead of an easy (and frankly hypocritical) hit of virtue signaling, maybe it's better to explain to them that they should keep their cool and take the time to judge accurately. Sometimes a book is indeed different from its cover, but other times the cover is exactly what you need to know. Ignoring abundant evidence from reality is not practical or Biblical. Case in point.
Its also a misapplication of the Fundamental Attribution Error, which is how humans have an extreme tendency to say "the reason me (or my friend) are late is because of external factors like traffic, the reason you are late is because you are a lazy, shitty person." As in, humans instantly judge a character based on often mostly assumptions, while giving a pass to themselves and those they know for the same problem.
Now, there are lazy, shitty people around, so its not 100% wrong. But you can also be lazy, shitty too. And even then only sometimes instead of it being your entire character forever. Which is where it becomes a major error.
"Don't judge" in fully defined and explained terms would be to avoiding snap judgements based on assumptions and predisposed biases, while also realizing that other people exist and also have circumstances (like traffic) instead of just waking up shitty.
But that's a long conversation filled with nuance and questionable moral choices, so its easier to simplify it for children, whose instincts will push them towards the opposite anyway.