Is D Vorah stronger than Mileena?
No. If you’re referring to the time when she killed Mileena, let me remind you, she killed Mileena when the latter was defenseless and restrained, not during in the middle of a fight. D Vorah is often tries to get the drop on characters when their guard is down or if they’re unable to defend themselves.
She is a deadly fighter, yes. Unfortunately, D Vorah is hardly much of an impressive fighter to begin with. Her fighting style isn’t based on skill or finesse, she just relies on her natural abilities and insects to do the work for her. The poisonous stingers makes her too dangerous to fight in close-quarters unless you’re capable of reacting quickly to her swift attacks.
In my opinion, if Mileena and D Vorah were to meet again and fought, I’m quite certain Mileena may pull off a win since she is aware of her own fate in her MK 11 bio. That doesn’t mean she wouldn’t have a hard time against the bug girl.