The Emerald Dream existed before Ysera was even born, she was charged with watching over it only once she became a aspect. She kept it safe for thousands of years, only time it was really threaten was when she was forced out, Awakened.
And I really wouldn't call a corrupted Pillar of Creation thrown into one's heart "corrupted easily". If she was never there the Nightmare would have spread a lot sooner with a far worse fate.