Depends on the level of illiteracy. Based on the way it works in the game, I figured that the trait reflected someone who was functionally illiterate, either due to terrible education or extreme dyslexia, with a reading speed of about one word per minute. Somebody like that could work out the labels on food and books, but actually reading even a magazine would be beyond them.
If you want a more brutal trait that represents somebody who can’t even recognise letters, then that might be worth another point and would require extra coding.