You Do Need power plug adapter while traveling from Monaco to Mexico.

Here is Why You Need An Adapter

The outlet and plug type used in Mexico are A and B and in Monaco are C, E and F. This is how they look like.

Monaco

In Monaco, outlets of type C, E and F are in use.

Type C

Type E

Type F

Mexico

In Mexico, outlets of type A and B are in use.

Type A

Type B