Trump explained it multiple times at rallies. (One has to keep monitoring the rallies as the stump speech does morph over time and you can miss things.)
He decided Joe was even more crooked than Killary, and he wanted to only use the 'crooked' label for one of them. Biden is the crook currently in his crosshairs, so Joe got the label.
Unstated, his advisors wanted him to back off from trashing women opponents, like his public feud with Rosie O'Donnell. They wanted the anti-woman label to stick to the 'what is a woman?' transgender supporter crowd instead of him.
He just calls her 'beautiful Hillary' because his supporters know he is being completely sarcastic when he says that.