It makes more sense in Britain to me than America, as I doubt anyone would remember what breed of cow was what in the post-war when it’s a mutated two-headed cow with no color but fleshy skin.
To me, it's the opposite. It makes more sense for Americans to name their mutated cows Brahmin, since Brahman is an American breed of cows especially bred to survive in low grazing areas, to be resistant to heat, diseases and parasites (ideal conditions to survive the wasteland). And they are heavily bred in America (and originated from Texas and the western Golf of Mexico). The Americans came up with the breed and named it all the way back in 1924. They even have an association around it (American Brahman Breeders Association).
Basically, Brahmin breeding is a huge thing in American cattle breeders.
Basically, Americans created the Brahman breed around the area where Fallout 1 happens.
Brahmin were named by people who survived the bombs. Fallout 1 was only 84 years after the apocalypse and everyone calls them Brahmin already.
Also, I don't really know if Indian people would name a cow Brahmin. After all, cows are the most sacred things in India and Brahmin are priests that are devoted to poverty or in other cases philosophers and wise men. It seems disrespectful to me to call sacred cows those things.
If the name came from Indian, or descendants from Indian people, it would be more likely that Brahmin would be called something like or related to "Gau Mata", "Kamadhenu", "Gau", "Pashu", "Surabhi" or "Shurbhi".
I also don't think anyone Indian or related to Indians would have anything to do with the post-apocalyptic US cows, since those are used as beasts of burden and slaughtered for meat. Which are great offences for Indian people.
I would have preferred the English version of Brahmin to be called something related to other English words (heifer, calf, milker, dairy, etc.).