Does Cull the Weak target?

No, Cull the Weak does not target.

According to the RiftJudge FAQ (ID 8519, 8598, 8613, 8910), Cull the Weak does not "choose" or "target" a unit when it is played. The instruction to kill a unit is performed by each player during the resolution of the spell, rather than being a targeted choice made when the spell is placed on the chain.

Because it does not target, it bypasses mechanics like Deflect and cannot be countered by cards like Not So Fast that specifically require targeting to function.