I've followed him from time to time over the course of probably a couple decades. He's slightly on the flamboyant side. To Get to the Truth, Senate Must Call Witnesses
DiGenova is a Washington insider, and who knows what sort of latent power Mueller has, etc. It also may be that DiGenova has bumped into Mueller in professional capacity, and found those interactions to be on the up and up. Assuming at least for the sake of argument that Mueller is a snake (I personally presume he is), it may be that DiGenova has never seen that side and has a mistaken impression, honestly held.
I also take the "Mueller is honest" claim from DiGenova as a throw away, meaning I don't take DiGenova's word for it and it doesn't matter wither way for purposes of what DiGenova is otherwise advancing.
One other thought. "Honest" doesn't necessarily mean "good judgment."
You won’t catch Mike Cernovich supporting Mueller!