No. You see, it's actually brilliant.

Chocolate milk is a liquid. A beautiful, tasty liquid. Water too, is a liquid.

Starlight didn't spill it. She returned the chocolate milk to its people. In essence, she made the chocolate milk larger. Diluted as all hell, but larger and beyond Twilight's reach.

Now, whenever the Purple Pony Princess gazes upon her school, she'll see the pond of chocolate milk-water that she can never touch, never spill. It will drive her mad.