Many of these types of words had functional use at one time. If you check Ellis Island immigration records you’ll see people classified as “idiots”, “imbeciles”, “morons”, and other similar terms based on an aptitude test administrated. Those 3 terms describe different levels of perceived intelligence.
Of course as they often do, these terms of classification became derogatory after people used them as an insult.
"idiot" may have been a relatively innocent medical term, but "mongoloid" has a very racist history as a way to describe people with Down Syndrome (https://en.wikipedia.org/wiki/Mongolian_idiocy), including a popular pseudoscientific theory that people with Down's were a result of "Caucasian" interbreeding with supposedly more primitive "Mongoloid" races https://en.wikipedia.org/wiki/The_Mongol_in_Our_Midst
Indeed and many of the terms, like “idiot” were pseudoscientific as well. Done and justified “by science” but this is not a condemnation of the scientific method. In fact, it further proves liberal science is the best known approach to knowledge and truth as it leaves open that bad science can be crushed by better science. I hope we continue this way however anti liberal science our politics are today.
Of course as they often do, these terms of classification became derogatory after people used them as an insult.