Do creatures with summoning sickness count as tapped?
Saying a creature has "summoning sickness" just means that it hasn't been under its controllers control since the beginning of their turn and it doesn't have haste. Such a creature cannot attack and cannot cause itself to become tapped (i.e. you can't pay a Tap or Untap cost in an ability on that creature - it can still become tapped or untapped by other sources).

A "tapped creature" simply means a creature which has been turned sideways for one reason or another.

In other words, Sheer Drop only cares about whether or not the creature has been turned sideways, it doesn't care at all about when the creature entered the battlefield. If your creature is sick but hasn't been tapped, Sheer Drop can't target it.