In most Austen novels we see how the characters who are less important are also described as "plain" or "not as beautiful" as the characters that serve as ornamental balances in contrast to the strong personality and psychological traits of the main female character. The word "plain" is about the equivalent of unattractive and is less used as a term than "not as pretty". In the "plain" characters we also see flat personalities and unimpressive characteristics. This can help us conclude that Austen assigned physical beauty to characters who were most passionate such as Sir Walter and less aesthetic importance to characters that led the story with their strength and balance such as Anne.
In Pride and Prejudice, for example, we see how Elizabeth is "not as pretty as Jane", who is ornamental in comparison to Elizabeth's strength of character and sense of balance. Yet, the sister with the most flat personality, Mary, is described as "plain".
Similarly the Dashwood sisters feature the pretty Marianne in contrast to the more sensible Elinor in Sense and Sensibility.
Conclusively, Austen allows the reader to create a bond with those characters described as "attractive" to perhaps enhance the romantic element of the story, and gives us cues as to the character traits of specifically flat characters by describing them as "plain".