This may be a stupid question, but whenever a character says, (or GRRM) has it written that a character is “simple” is that their way of saying that that character is mentally retarded/handicapped? People like Helicent Uffering, Jaehaera Targaryen, Gael Targaryen, and “Jinglebell” Frey, etc.?

I’m not an expert in the surprisingly voluminous subfield of mental health in the Middle Ages, but if I recall correctly from what I’ve read, it could be some form of handicap, it could be mental illness, it could be trauma, it could be general non-confirmity of behavior.  

And it gets doubly complicated in GRRM’s world, because of the way that magic intersects with all of this. Prophecy is definitely something that can seriously damage the mind, as we see with Patchface. So it’s quite possible that some Targaryens known to be “simple” – thinking of Rhaegel, for example – might have inherited the family gift for prophecy too strongly.