It can be used in several different ways:
* Genuine sympathy: "Woe is you, you lost your keys again!"
* Sarcastic pity: "Woe is you, you had to work late. Imagine that."
* Mockery: "Woe is you, you didn't get invited to the party. Maybe if you weren't so boring..."
The key takeaway is that "woe is you" is a dramatic and often hyperbolic way of expressing feelings about someone's misfortune.