I've talked about this before in the other thread.
Ulti controlled people, such as Squall, Seifer, etc as part of her plan.
Part of her plan was that when she was defeated, the affected minds will be restored into happy moods and act like nothing bad have happened. That's why everybody got happy after Ulti's defeat.
In reality, there will be next Ulti in the future, they still haven't realized about that.