Targum Pseudo-Jonathan on (Genesis 5:3) reopens old wounds. "Adam lived a hundred and thirty years, and begat Sheth, who had the likeness of his image and of his similitude: for before had Hava born Kain, who was not like to him."
Cain did not look like his father. The Targumist is pointing at a tradition — picked up later in the Zohar and other mystical sources — that Cain was not entirely Adam's son in the spiritual sense. Some readings suggest he carried traces of Samael, the angel Eve saw behind the serpent. Cain was, in this reading, less than fully human in his likeness to Adam's image.
"And Habel was killed by his hand. And Kain was cast out; neither is his seed genealogized in the book of the genealogy of Adam. But afterwards there was born one like him, and he called his name Sheth."
The Targumist is making an editorial point about the Torah itself. The genealogy of Genesis 5 skips Cain entirely. The book of Adam's descendants begins with Seth, not Cain. Cain's line is recorded in chapter 4, but not in the covenantal genealogy. Seth is the true heir, the first son who bore Adam's full image — and through Seth runs the line that leads eventually to Noah, Abraham, and David.