[Suicide Junkie]

If you're going to throw a black hole, it'll have to be dang heavy to do any more than poke a molecule-sized hole through the target...
Gravity is the weakest force, after all.

A hole with the mass of a mountain is going to be worse than microscopic, and have no appreciable (ie hullbending) gravity field on macroscopic scales.

Instead of wasting all that energy on a shot that goes right through (hoping to hit the captain in the head, or the main reactor say), you'd be better off throwing that mountain uncompressed with the same energy, and thus flatten the entire enemy ship.