Satire uses humor to poke fun at human and societal weaknesses. It can be very dark, as in Swift's "A Modest Proposal," but also light-hearted. Fielding chooses the light-hearted path in this novel.
For example, though title character Tom Jones has both good and bad traits, we like him because the traits in him that are satirized have a forgivable quality.
For instance, Tom is the ultimate party animal: he loves drinking, good times, and women. He is outgoing, sometimes a bit arrogant, and yet always open-hearted. In fact, he gets himself into trouble because he is so uncalculating in his self-presentation, not realizing how his behavior can and will be twisted by unscrupulous people.
For example, we can't help but sympathize with Tom when, though he does overindulge in drink and partying, he does it to celebrate Squire Allworthy winning out against death—and we feel more on Tom's side when Blifil unfairly twists this goodwill to Squire Allworthy so that the squire kicks Tom out of the house.
In the same vein, we can condemn Tom for his womanizing—and he does make his mistakes—but he does also truly love Sophia. He is uncalculating in his affection for her, and when he does try to "calculate" to win her, he does so in ways that underscore how completely hopeless this good-natured man is at scheming.
We also tend to like Tom because his flaws are so much more preferable to Blifel's flaws of hypocrisy, back-stabbing, and calculation. In putting these two characters side by side, Fielding encourages us to interrogate a society that heaps moral condemnation on what might be the least contemptible of vices while overlooking true moral vileness. As Fielding's narrator comments, it is wrong to "condemn a character as a bad one, because it is not a perfectly good one."