This quote speaks directly to the PR nightmare that is the NSA. With every new leak, the United States damages another relationship. The United States' citizens and allies will only take so much more. If the U.S. does not gain control of this story, it is likely to face internal and external hostility at every level for years.
This quote is just consequential after the Snowden incident and increasing doubt from other governments regarding US surveillance. This isn't a relationship the United States can step away from. Being a neighbor, and one of the top traders to the US, the Mexican foreign Ministry is right to address this problem.