Can I Smoke Screen my opponent's unit that is the target of Baited Hook before it is killed?

Yes, you can.

Smoke Screen has [Reaction], so you can play it in response to Baited Hook's activation while the ability is on the Chain (Closed State). It resolves first (LIFO) and reduces the targeted unit's Might before Baited Hook kills it.

What happens: Baited Hook checks the unit's current Might at the moment it is killed (FAQ #2156, FAQ #8885). Since Smoke Screen reduces that value — to a minimum of 1 — the opponent's search threshold becomes "Might up to 1 more than" that reduced value. If you drop a 4-Might unit to 1, they can only fetch a unit of Might 2 or less instead of 5 or less.

Important: The unit must still be a legal target for Smoke Screen at the time you play it — it needs to be on the board and meet the "unit" targeting requirements. Baited Hook's target is still on the board when the Chain is open for Reactions, so that's satisfied.