The concept Fielding describes at length in his Preface is affectation (not "affection"). Merriam-Webster defines it as the following:
1a: speech or conduct not natural to oneself : an unnatural form of behavior meant especially to impress others. b : the act of taking on or displaying an attitude or mode of behavior not natural to oneself or not genuinely felt. 2 obsolete : a striving after.
Definition (2) was either already obsolete by Fielding's time or was, in any event, not what he meant by the term as he expounded it in his Joseph Andrews Preface and used it in the novel itself.
Fielding's story is largely a satire on Richardson's Pamela, which Fielding had already more directly parodied in his novella Shamela a year earlier (1741). In Joseph Andrews Fielding presents to us different types or degrees of affectation as a means of propelling his story-line and getting his themes across. Mrs. Slipslop is the first example, with her pretentious use of words and her lording it over Mr. Adams because she presumably knows more than he does:
She professed a great regard for his [Adams's] learning, and would frequently dispute with him on points of theology; but always insisted on a deference to be paid to her understanding, as she had been frequently at London, and knew more of the world than a country parson could pretend to.
Adams therefore took an opportunity one day, after a pretty long discourse with her on the essence (or, as she pleased to term it, the incence ) of matter, to mention the case of young Andrews...
Mrs. Slipslop constantly uses malapropisms, where a like-sounding, sometimes invented word is substituted for a correct word, as "incence" is in this quote for "essence." Both her own assumption of superiority over Mr. Adams, and her attempt to impress him by using elevated-sounding words (and doing so incorrectly) are instances of affectation. Joseph himself is the virtuous counterpart to the virtuous Pamela (his sister) in Richardson's novel. His behavior seems to be affected, but he presumably is actually as unrealistically innocent as he acts. For instance, he becomes a connoisseur of opera.
Given the usual dismissive attitude English writers of this period have about that "exotic and irrational" form of entertainment, one would think Fielding is skewering Joseph's attitude (and that of the other footmen who follow Joseph's lead on this), and that his enthusiasm for opera is another thing that is obviously an affectation. But is it? Both his employer Lady Booby's behavior towards him (or her way of "explaining" her behavior) and his reactions to her are examples of what, in any realistic world, would have to be false and artificial. After the death of her husband, Lady Booby calls Joseph into her room, where she is lying in bed naked (as we soon find out) with nothing but a sheet over her:
"La!" says she, in an affected surprize, "what am I doing? I have trusted myself with a man alone, naked in bed; suppose you should have any wicked intentions on my honour; how should I defend myself?" Joseph protested that he never had the least evil design against her.
Lady Booby, of course, knows exactly what she is doing. Joseph's reactions are a parody of Pamela's resistance to a man's attempt to force himself upon her, but when the gender switch is made and a woman makes advances to Joseph, his behavior seems a comical affectation.
The technique of Fielding in Joseph Andrew is one where, in the usual way that satire works, something judged hypocritical or affected is presented in an exaggerated way and shown as ridiculous. Fielding is deriding affected behavior among people in general, but more specifically what is depicted in Richardson's Pamela, and the style and overwrought moral message of Richardson's novel as a whole. So, we can see how the concept of affectation is used for comic affect and also to make a serious point about human nature and frailty—and about literature as well.