This is an awesome question to pit the stock characters of Volpone against the characters in The Duchess of Malfi. The female characters to be admired are Celia and the Duchess, while the ones we are left to ridicule are Lady Politic Would-Be as well as Julia and Cariola.
Celia in Volpone and The Duchess of Malfi in the piece named for her are the two females who the reader is meant to admire. These are the two virtuous female characters that are easily compared. Celia, of course, is the "virtuous woman" as compared to other characters given nasty nicknames in Volpone. Celia is gorgeous and, even though presented with both peril and slander, she always reacts with fortitude. Her character is foiled by her jealous, horrid husband, Corvino (which, of course, means "Crow") who is a mean merchant, cowardly by nature, and incredibly jealous of his pretty wife. The reader is meant to be completely repulsed. This makes Celia's beauty and fortitude all that much more attractive. Similarly, we have the very noble Duchess of Malfi in the story named for her. And with the Duchess, we have a good quote that could truly exemplify both women:
Integrity of life is fame's best friend,
Which nobly, beyond death, shall crown the end.
There is a very good reason for this compliment of the Duchess: she is the ONLY character who is independent, self-reliant, and strong. In this way, she could be used to contrast the male characters as well! Envy and fear and lust have no place with the Duchess. Even Antonio, who is noble enough to be of interest to the Duchess, holds back while the Duchess isn't afraid to push forward. Where females were supposed to be submissive, the Duchess is most certainly not. In fact, refusing to do the will of the villains of the story, she stands alone in virtue... even though it means her death. In fact, her strength is exemplified when she says, “I am Duchess of Malfi still.” One interesting thing to note about the Duchess is that she chooses to marry someone not in her same class. Therefore, even in the way she doesn't adhere to the usual constraints of women she is still virtuous.
On the other hand, we have female characters who we are NOT generally meant to admire. In Volpone we have "Lady Politic Would-Be," a stock character if we've ever seen one. She is absolutely repulsive, and an easy female foil to Celia. This Englishwoman is simply an echo or a parrot with a grating voice and a pea-size brain. Volpone can't STAND her, so much so that even as the sly fox he is unwilling to be subjected to her character regardless of the financial gain she might offer! The euphemism used by Volpone is as follows:
[Lady Politic Would-Be is] my madam with the everlasting voice.
Lady Politic Would-Be is as jealous as the jealous Corvino and makes herself of use to Mosca because of her false testimony. Finally, let's look at the more minor characters in The Duchess of Malfi. Julia and Cariola are meant to be foils or direct contrasts to the Duchess. Where both the Duchess and Julia are strong enough to choose the men of interest to them, Julia uses her abilities to seduce Bosola instead of to honor. In fact, she gets her comeuppance when the Cardinal offers her the poisoned book. Julia kisses that book and meets her fate due to her dishonest nature. We also have the Character of Cariola who, even though she is a very minor character, is also a female. Cariola is the Duchess' servant. She is feeble-minded and also lower in class. She simply strikes a contrast with the Duchess' true nobility.
In conclusion, it's important to realize that, no matter what female we are speaking of, each character remains pretty much the same during the story. In other words, what is interesting is that although Celia and the Duchess are incredibly admirable, they are still fairly flat characters. Their nobility and virtue are so strong from the beginning that there is no need for them to change during the story.