“Wasn’t he born in Mexico?”
No hid father George was born in mexico to US citizen parents making him a US citizen. (George was not an NBS but that is not necessary for Mitt to be a NBC...born in country to citizen parents........is the standard for president.
I doubt his parents being us citizens would help because George’s parents had taken up permanent residence in Mexico. This is different from US Citizens stationed temporary do to some type of service. Military, corporate etc, or simply on vacation.
The question is when did George get nationalized. If before Mitt was born then OK. If not then he is not NBC.
George was listed in a census as a US Citizen by his father so he was probably never actually naturalized because his father always considered him an American.