No. There are a lot of different versions of this question. Was X a proto-communist/capitalist/gay rights activist/civil rights activist, etc? The problem is that you're taking a word that represents a specific ideology which developed in a very specific historical moment. In the case of feminism, the late-nineteenth and twentieth centuries. I personally am very cautious of labeling things from the Middle Ages as proto-twentieth century ideologies. But there are two tests that we could put this to. First, does the person or work in question explicitly subscribe to at least some of the ideologies of Feminism? Second, if they did not explicitly subscribe to them, were they trying to work toward them in some way, either consciously or unconsciously?

In order to answer this, we need to know what feminism is. Now, you may be aware that a definition of feminism is a very complicated thing to come by, as it can mean different things to different people. You have suggested that the Decameron contains at least two feminist themes, sexual freedom and the strength of women. However, this is not a strong definition of feminism. In fact, sexual freedom is something that has been constructed differently by different people claiming to be feminists (sometimes even being mostly ignored). And depictions of strong women are not necessarily indications of feminism at all. I wouldn't call men in classical Athens feminists, but I would call Euripides' Medea a depiction of a strong woman.

A better definition of feminism is that it is an ideology which seeks political, personal, legal, and social equality for women. This is probably not a perfect definition, and there are definitely different ways that self-identifying feminists have defined and sought equality. But that is generally what feminism is.

Now, does the Decameron overtly support equality for women? No. It is totally unconcerned with challenging the traditional place of women in society on basically any front. So it fails the first test.

The second test, is it testing the boundaries of female equality, even unconsciously? The answer is again no. Viewed within the context of medieval society, Boccaccio's depictions of female characters reinforce misogyny and women's position in society.

For example, we understand sexuality within a modern lens. For the most part, people in western society have begun to celebrate sexuality -- although feminists and others can still critique modern attitudes toward women within it. In the Middle Ages, although in some ways they were far more open about sexual activity than we are, they also used sexuality as a way to critique people. The official and unofficial doctrines of the Catholic Church emphasized chastity for all people. Sexuality was disordered because of the fall of man, and it should only be resorted to in order to procreate. Pleasurable sex was to be avoided, and sexual urges were to be controlled. Although medieval lay people didn't adopt adopt this version of sexuality whole-hog, that was still one of the guiding ways that sexuality was understood.

There was also a concept that we sometimes refer to as the faculties (or the souls), which was how medieval people understood the differences between the body and the mind. This is actually still, to some extent, alive as the Mind-Body problem and questions about what separates us from animals, but it originated with the Greek philosophers. To massively over simplify it, people believed that living things had several different faculties. The physical, governed by the body (matter), and the mental, which, properly ordered, would be governed by reason. Plants had only the physical. Animals had the physical and the mental passions, but lacked reason. And humans had all three. However, most humans were still governed by their passions. Only a few had truly learned to overcome them.

Now, consider Boccaccio's sexually voracious women. They are people who cannot control their sexuality to the extreme (consider his convent of lusty nuns!). In contrast to men, this says that women are overly controlled by their physical desires and passions, whereas their reason is suppressed. This is not a flattering portrayal of women. It is certainly not one that would argue that women, ruled by body and emotion rather than mind, should be more equal in society.