Before. Joseph Smith died in 1844, and Henry Goddard didn’t coin the word “moron” for medical use until 1910 (after which it quickly became a pejorative term).

I think Smith was trying to come up with vaguely “Semitic” sounding names, like Nephi, Lehi, etc. but linguistically the whole thing falls apart. The Bible has issues, but outside of the mythological parts, one can take it as a fairly accurate history. The Book of Mormon is just straight up fiction.

The angel Moroni was declared by Smith in 1823. The term moron was coined by Henry H Goddard in 1910. So, it’s unlikely that Smith had ever heard of the word moron before naming the angel

The terms Moron, Imbecile, and Idiot were replaced with Mild, Moderate, and Severe mental retardation—which have also become pejorative,

The Book of Mormon was also written in a very bad rendition of Elizabethan English, to sound like the King James Bible, in spite of being written in the early 19th century.

Before.
According to Merriam-Webster’s Collegiate Dictionary, 11 ed., the term “moron” came into being in 1910.