When you recruit Kimberly, the other characters seem incredulous at the fact that she's illiterate, but I think this actually makes a lot of sense. A literate person would have preconceived notions of what letters look like, which could subconsciously affect your forging abilities. An illiterate person wouldn't have such biases, and would therefore be more likely to copy the letters exactly as they are in the reference that she uses to forge from.
"I'm a deeply flawed individual, but I bring joy to the world." -Tequila Sunset