Star Trek: Discovery‘s lonely ready room Tribble might have not just been there to show Lorca could resist its cuteness, but to Lorca the resistance may have been the point.
We eventually learn that it that Lorca maneuvered himself into Discovery’s captain’s chair specifically because he understood its potential to take him home to the Mirror Universe. He abandons ship as soon as the ship arrives home, leaving the Discovery crew with no obvious way to get home.
That means as soon as we meet him in Star Trek: Discovery, Lorca is planning something that he knows is likely to lead to the exile–if not the brutal deaths–of a crew that has come to trust him. He’ll need thick skin to do something like that–even skin thick for a Terran.
The Tribble may have been Lorca’s way of preparing himself emotionally for the betrayal. In other words, the constant starvation of the Tribble we see–and quite possibly the occasional slaughtering of baby Tribbles we never see–may have been a way a way for Lorca to flex his cruelty “muscles,” for lack of a better word.