> "Día de los muertos" sounds so weird to me, like we're talking about a particular group of dead people (say, the dead from a war) instead of the dead in general. I would say "día de muertos" and most Mexicans in Mexico agree.

I'm sure all Mexicans would agree. Saying "día de los muertos" sounds like calling July 4th "day of the independence". Maybe it's a literal translation from "day of the dead", since "dead day" sounds like something else.