SHATNER, the actor, doesn't have the capability (especially at his age and weight) to threaten a field mouse. You put him on screen and everyone is just going to laugh at him if he tries to act initmidating. Regardless of what "character" he is trying to play.
You should see THE INTRUDER, if you haven't already. (Or, if you're in the mood for some raw '70's cheese, IMPULSE.) Shatner can be one creepy weirdo if a director knows how to channel his quirky screen presence.
Anyway, I actually pictured the Abrams-crew displaying the exact same open mockery towards this old, fat clown within the context of the film. They laugh at him, they lower their defences, and then...BAM, Tiberius suddenly thrusts a blade right into McCoy's belly.