Is a tapped creature an illegal target?
Flesh to Dust says "target creature", so the target must be a creature. Period. Whether it's tapped or not is of no consequence. Same goes for Akroan Mastiff's ability; it simply won't do anything on resolution if the creature is still tapped then.

Hexproof, Shroud, Protection and other abilities can impose additional limits. Don't forget that creatures can be granted abilities (e.g. Lightning Greaves), and that creatures can be the subject of continuous effects created by abilities of other permanents.