One of the themes of Tom Jones is appearance vs. reality. Using this theme, Fielding exposes what he sees as the hypocrisy of contemporary English society. The character of Lady Bellaston is a prime example of this. On the face of it, she's a respectable aristocrat, a lady of quality. Yet in actual fact, she has no real sense of propriety at all, engaging in numerous affairs, treating Tom as a gigolo, and openly expressing her contempt and loathing for the institution of marriage.
Fielding is suggesting that the appearance of respectability is used by the aristocracy to conceal the reality of their rampant selfishness and immorality. Though Fielding doesn't idealize the lower classes, it's telling that some of the most sympathetic characters in the story are simple folk such as Black George the gamekeeper. Lady Bellaston and her fashionable London set would doubtless look down on such people, yet they are the ones who are genuinely respectable in their behavior and how they live their lives. But society's self-appointed leaders are too shallow, too obsessed with outward status and appearance to understand this.