You really should be celebrating as your Dollar is buying MORE pesos per dollar and this tends to insulate you somewhat from the inflation that results from buying things in Pesos.
This is GOOD for you unless you make your INCOME in pesos.
Now this does mean that some things in Mexico will be more expensive in Peso terms but not that much.
Keeping hundreds of dollars in pesos when you return to the US is foolish. You are basically betting that the peso will strengthen against the dollar...which is not going to happen.
Use up your pesos on the way back out.
I have spent a good deal of my working life living traveling and working overseas using a multitude of currencies. The Peso is actually one of the better ones....just be thankful you are not using the Lao Kip where $100 get you about 1.2 MILLION Kip, or the Cambodian Reil or the Vietnamese Dong.
The farther down the peso goes in value the more you get for one dollar the cheaper overall it is to travel in Mexico.