Appearances are almost always inversely related to the actual nature of the characters in Jane Eyre. Beautiful women turn out to be scheming harpies or selfish idiots; plain women turn out to have hidden depths of passion; ugly men aren’t actually ugly, but excitingly masculine in a harsh, craggy way. Virtuous characters resist having their appearances radically changed or improved because doing so seems like pretending to be something they aren’t. In contrast, characters who let themselves get obsessed with keeping external appearances plain and modest are distracted from deeper spiritual truths and often turn out to be hypocritical anyway.
Jane’s claim that she is a "plain, Quakerish governess" is actually somewhat misleading; her attempt to hide her beauty from the reader demonstrates her belief that beauty and morality are mutually exclusive.
Although other characters occasionally claim that Jane is beautiful, her beauty is always related to her mood or her character; it’s an "inner beauty" that the reader can only understand because Jane is a "plain, Quakerish governess" on the outside.