Joseph has a particularly pretty birthmark on his chest. Now, it's not like he goes around flashing that birthmark to everyone, but still, all of Joseph's buddies know that this birthmark separates Joseph from the pack. Little do they know that it's also the thing that shows his true parentage—yep, it turns out that Joseph is a gentleman, through and through. As Mr. Wilson's son and heir, he's got plenty of wealth coming to him.
Joseph is a handsome guy, but he's by no means perfect. Fielding takes great pains to show how Joseph is always in the middle. He's middle-class, middling height, and not particularly smart about navigating the world. Joseph's flaws help him, though. The strawberry-shaped birthmark is the perfect example of a flaw that changes Joseph's life for the better, due to how recognizable it is.
Take a look at what the peddler says about Joseph and his birthmark, for example. After all, it's the savvy peddler who brings the birthmark to everyone's attention. He asks Gammar Andrews if her supposed kid has a birthmark, and she answers: "Yes, he had as fine a strawberry as ever grew in a garden" (4.15.4). By implying that Joseph's strawberry birthmark is natural, Gammar Andrews suggests that he's pretty awesome, flaws and all.
See, even if Joseph isn't at the top of his game, his birthmark shows that he's a natural gentleman. We're thinking that Fielding is making a joke about how virtue is inherited. Is it really passed down from generation to generation, like social status? We mean, really, if it all comes down to a fruity birthmark, then how important could any of this be? Well, hey: better check for your strawberry-shaped birthmarks, Shmoopers—you could be kings in disguise.