If the target creature is an illegal target as Dreamdew Entrancer’s last ability tries to resolve, it won’t resolve and none of its effects will happen. You won’t put stun counters on the creature, and you won’t draw cards no matter who controls it.

You may target a creature that is already tapped with Dreamdew Entrancer’s last ability. If the target creature is already tapped as the ability resolves, you’ll still put three stun counters on it, and if you control it, you’ll still draw two cards.