I found these rules (in Spanish, naturally):
I speak conversational Spanish reasonably well, but I'm not confident in my ability to translate legalese (It's hard enough for me to translate USA legalese into plain English). Those two documents contain content which is similar, but not exactly the same. I don't know if one is a later revision than the other, or what. I don't know which one would be more authoritative.
5.1.1(a) of the first link says you have to register drones weighing more than 0.250 kg (250g). I can find no registration requirement for drones lighter than that.
4.7.18 of the second link says that a UAS with foreign registration or operated by foreign operators, other than those mentioned in 4.7.17 of this Official Mexican Standard, may not operate in Mexico, unless there is a bilateral agreement between the Aeronautical Authority and the Civil Aviation Authority of the state of registration.
Since it's referenced above, 4.7.17 says that a foreign-registered UAS operated for scientific purposes needs the permission of the Ministry of National Defense.
I'm not sure if 4.7.18 applies to sub-250g drones, since they don't seem to require registration to begin with, but I really wouldn't want to argue the point in a Mexican police station or courthouse.
There are a lot of miscellaneous airspace and operating regulations, some similar to the US, but plenty are different. There is a requirement to read all NOTAMS (4.7.6), and to avoid dangerous, prohibited, or restricted areas as listed in PIA/AIP, section ENR 5.1. (4.7.5). Since meeting both of those requirements would require some ability to decode Spanish, I'm going to stop here, and let the prospective pilot figure out all the details. And while you're at it, get a copy of the relevant charts of airspace and airports.