Depends on how in control Dio was, my theory is that Versace and Giorno were the last of Dio's children and as such came out fairly normal lookin due to the fact that Dio was more melded with Johnathan's body and thus passed on a more stable meld of his and Johnathan's genes. Contrast this with Ungallo and Rykiel who look as though they have some sort of defect ( Granted, heavy drug use could be responsible for Ungallo looking like Sloth from the Goonies). They were concieved earlier and thus recieved a tepid genetic cocktail of Brado/Joestar DNA, that for all intents and purposes, would produce children with facial related birth defects or similar mental and psychological defects. Also, since none of Dio's children ever show signs of being Dhamphirs, we can assume that they got more God-tier Joestar genes than they did Brando fail-genes