The older Mukuro will not take kindly to the intrusion upon his thoughts/feelings, and if he doesn't simply shut himself up even tighter then he'll actively push/hurt the invading mind until it retreats. He doesn't like having people rummaging through his head, thank you very much, though he might skip over inflicting actual mental pain if he understands that Callo can't help doing it. Expect him to be annoyed if she can't find a way to reign it in after some time though. If she's lucky and approaches him right, he might even show her how to.

(All that said, I'm open to her trying to read him. Just don't expect it to be successful. I'll detail more about what she might see if the situation calls for it.)