There are no current restrictions for travel into Mexico as it is “fully open”.
However, if you are traveling through the US to get to Mexico you must be fully vaccinated with the primary series of an accepted COVID-19 vaccine and are also required to show a negative COVID-19 test result (within 24 hours of departure to the US) or documentation of recovery from COVID-19 when you travel to the United States by air. Wearing a mask over your nose and mouth is required in indoor areas of public transportation (including airplanes) traveling into, within, or out of the United States and indoors in U.S. transportation hubs (including airports).
The U.S. accepts the use of rapid antigen tests administered by a lab or health-care provider for entry, although more expensive molecular tests such as PCR tests can also be used. At-home rapid antigen tests can only be used if the test is taken under the supervision of a telehealth provider through an audio and video connection. The test must also be approved for emergency use by the U.S. Food and Drug Administration or by the “relevant national authority where the test is administered”.
Note: If you fly into San Diego and cross the border to Mexico by land, there is no testing requirement when you return across the border. See https://www.dhs.gov/news/2021/10/29/fact-sheet-guidance-travelers-enter-us-land-ports-entry-and-ferry-terminals for further details.