Some main points to consider in an analysis of Mosca:
1) Mosca is an amoral opportunist who uses the art of deception for his own gain.
In the play, Mosca is everything his name implies about his character. Like a fly which feeds on rotting flesh, Mosca is not particularly concerned about who his victims are. He is as comfortable gulling Corbaccio, Voltore, and Corvino (whose names mean "raven," "vulture," and "crow" respectively) for his own gain as he is his own master, Volpone.
In the story, Mosca helps Volpone to trick the three legacy hunters. Each of the three are like carrion birds, circling their victim, Volpone, as he supposedly dies an early death. Anxious to be bequeathed his fortune, all three eagerly enlist the help of Mosca to secure their rights to it. However, unknown to all three, Mosca is playing them for fools. For example, Mosca tells Voltore that he is Volpone's sole heir (which is not true). When Voltore asks how he came to be chosen Volpone's heir, the wily Mosca answers that Volpone has ever admired lawyers, men who can "speak to every cause" and who can "give forked counsel." To top it off, Mosca even begs the unscrupulous Voltore to remember him when he comes to "swim in golden lard, up to the arms in honey."
Another instance of Mosca's amoral tendencies is when he plots with Corvino to force his virtuous wife, the beauteous Celia, to sleep with Volpone. This brings us to another point about Mosca:
2) His temperament is perfectly suited to his tendencies.
When Corvino mercilessly threatens his wife, Celia, with the worst punishments for being less than enthusiastic about sleeping with Volpone, Mosca doesn't bat an eye. He freely claims that Corvino is here to prostitute Celia, and he's perfectly fine with that. Here, Mosca is working both sides; while claiming to work on behalf of his master, Volpone, he also humors the despicable Corvino. In front of Volpone, Mosca claims that Corvino is offering his wife "freely, unask'd, or unintreated." That, of course, is a lie. It is Mosca who has actually put Corvino up to offering his wife as a sacrifice of sorts.
Meanwhile, the faithful Celia begs to be spared such a disgrace, but her husband is resolute. He tells her that he will rip up her mouth to her ears, slit her nose, and proclaim her a whore on the streets if she doesn't go through with the act. Mosca, the parasite, looks on and benignly tells Celia to comply. His cool nonchalance is evidence that he is a man who can rein in his emotions when he needs to. Not for him is the sensual indulgence of emotion so prized by his employer, Volpone.
In fact, Mosca is able to craft the narrative to fit each situation neatly. He is able to juggle each story with consummate skill and artistry. More than anything, his iron control over his emotions is key to indulging his amoral tendencies.
3) Mosca's avarice is fueled by self-interest and self-interest alone.
While Volpone is focused upon hedonistic pleasures for their own sake, Mosca is concerned about the final results of his machinations. He always seeks to gain from every interaction. His position as a parasite is precarious; so, in his mind, he must agitate for his own benefit because he is the only one who cares about his future. However, Mosca's amoral tendencies as a master of deception results in his ruin. In appropriating his master's property and wealth by using a supposedly legal will (ironically signed by Volpone), Mosca inspires Volpone's wrath. It is Volpone who eventually admits everything to the judges; finally, because of his part in all "these lewd impostures," Mosca is sentenced to be whipped and to spend the rest of his life in perpetual slavery.
Source: Volpone and Mosca: Two Styles of Roguery
Mosca is a complex character: he is on the one hand a parasite (Mosca means "fly") who feeds off the other characters. He plots with Volpone to trick the fortune hunters into delivering gifts in hopes of winning Volpone's fortune. He also schemes with Volpone to get his master alone with Celia and create an opportunity to take advantage of her.
Though Mosca's actions are by no means "good", his cleverness in creating schemes that get him what he wants demonstrates an intelligence and superiority over the other characters. Mosca largely takes advantage of other parasites (like his master), claiming that
All the wise world is little else, in nature,
But parasites, or sub-parasites.
Mosca also shows that deception is an art: his actions drive the plot of the play forward and the audience no doubt takes a certain delight and pleasure from watching his tricks unfold.