Yes, flickering a creature fully resets any damage marked on it. When a creature is exiled, it ceases to exist on the battlefield as a game object. When it returns from exile (usually at end of turn or immediately, depending on the effect), it enters the battlefield as a brand-new game object with no relation to the previous one.
This is governed by CR 400.7, which states that an object that moves from one zone to another becomes a new object with no memory of or relation to its previous existence. Marked damage is not a property that travels with an object — it exists only on the permanent while it is on the battlefield (CR 119.3).
Additionally, state-based actions check whether lethal damage is marked on a creature (CR 704.5g). Since the returned creature has no damage marked on it, it survives even if it had previously taken lethal damage before being flickered.
Example: Your 2/2 creature has been dealt 2 damage (lethal) this turn. In response to it dying, you cast Cloudshift, exiling and immediately returning it. The creature re-enters as a new object with 0 damage marked — it is a fresh 2/2 and survives.
Note that any Auras, Equipment, or counters that were on the creature do not automatically return with it unless a specific effect says so (CR 400.7). The creature re-enters clean in all respects unless the flickering spell explicitly restores those things.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.