Remember that English words do not necessarily align perfectly with the meaning of particular Greek words used in the original text. The original Greek word used here is "moros", which I presume is possibly where we get the word "moron". It means foolish, impious, godless, without learning, useless. It is a very strong word, and may have had more negative implications than our English word "fool" which it has been translated as. By contrast the word "raca" used earlier in the verse with less condemnation means empty-headed, so is less strong.
I think we shouldn't be worried about the particular word, but rather the sentiment behind it.