SBF chose to make his character a part of his defense ("Whoopsie daisies I'm just an incompetent nerd who likes muffins and didn't know I was doing anything wrong"), which makes character attacks like that fair game against him for the prosecution.

This is precisely why you shut up, and plead the fifth. You'll be eviscerated on cross-examination when you don't.

Although in his case, he probably should have pled guilty, and begged for mercy, instead.