You can infer, though not show directly, that Jesus was likely to have thought of mental retardation as demon possession due to the range of non-mental infirmities that were described as being caused by demons.
In Luke 13:10-13 Jesus is said to have healed a crippled hunchback woman "...who had been crippled by a spirit for eighteen years. She was bent over and could not straighten up at all.".
In Matthew 12:22 it is said, "Then they brought him a demon-possessed man who was blind and mute, and Jesus healed him, so that he could both talk and see."
And in Luke 11:14, "Jesus was driving out a demon that was mute. When the demon left, the man who had been mute spoke, and the crowd was amazed."
If grossly physical ailments could be considered to be demonic then anything, including retardation, would be most likely to have been thrown in with the rest.