You are asking two questions: efficiency of regulator into a short circuit and power loss in regulator in short-circuit condition.
The first is easy:
Efficiency is give by formula \$ \eta = \frac {P_{OUT}}{P_{IN}} \$ and power can be calculated from \$ P = VI \$.
For the short-circuit case \$ V_{OUT} = 0 \$ so we have
$$ \eta = \frac {P_{OUT}}{P_{IN}} = \frac {V_{OUT}I_{OUT}}{V_{IN}I_{IN}} = \frac {0 I_{OUT}}{V_{IN}I_{IN}} = 0 $$