Having had a "decnet education" only means that in spite of her low origins (as Anne puts it -she's pretty blunt about it) she had been sent to a ladies' school and learned the basics of genteel behaviour and had soem education. But like Lucy Steele, she clealy wasn't more than moderately refined and not very well educated as such. Lucy is clever and has managed to cover up her vulgarity to an extent but she's "illiterate" ie not well read or inot reading for its own sake.