I must have downloaded hundreds of birth, marriage and death certificates from a much later period than this, especially of people in the east-coast fishing villages, and well into the 19th c. many women in the fishing villages could only make an x on their marriage-certificates, or the grandmother would make an x when registering a birth in the family. But that's not to say that there weren't plenty of exceptions, and one should never generalise about such things, I agree with you there.
Harry