Screech decreases the target's Defense stat by two stat levels.
Screech will do nothing if the target's Defense stat has reached a stat level of -6 or a value of 1, and will fail if the target has a Substitute or is under the effect of Mist. The effect of Screech is ignored by critical hits.
Does this mean a crit could in fact, end up dealing less damage than a normal hit?
They fixed that in Gen III onwards (maybe Gen II). It was true in Gen I, though: you'd do 2x unmodified damage.