Is the song "Que Sera Sera" about death?

Quite the opposite. Nothing is certain EXCEPT death...and taxes. The song lyrics advises to accept "what will be, will be" in our lives because no one can predict what will happen in the future since fate is not in our hands. It's meant to be a LIGHTHEARTED song, so don't read into it anymore than is already there.
http://www.azlyrics.com/lyrics/dorisday/queserasera.html

This was a huge POP hit for singer/actress Doris Day, and she sang it in several movies, as well as releasing it as a record. It was featured first in the Alfred Hitchcock film "The Man Who Knew Too Much" (the 1956 remake of Hitchcock's earlier version), where the character of the mother is changed from a British clay pigeon shooting competitor to a famous American singer, thereby giving them an excuse to write a musical number into a non-musical movie. They did manage to work it into the film as an important plot device.

https://www.youtube.com/watch?v=zWRmaPFdlx4&ab_channel=CurlyB
https://www.youtube.com/watch?v=vsMUEMSlUlc&ab_channel=mm954


Actually,there was some sort of "classical" music in both film versions---"The Storm Cloud Cantata" which was specially composed for the film. It still served not only to provide logical music for people to be listening to in the concert hall (and they could have very well used an existing classical work), but it also heighten suspense in key scenes leading up to a needed (for the plot) cymbal crash and/or main character screaming and thus diverting an assasination attempt.
https://en.wikipedia.org/wiki/Storm_Clouds_Cantata

No. It's about wanting answers to life. Que Sera Sera, means Whatever will be will be. In other words, you have to wait and see. Good advice.

Did you read or hear the rest of the lyrics? They make it pretty clear what it's about.