I have been looking at the following two questions recently:
The first states the following in the question itself:
[...] In addition to such spells, there are more general effects that cause death, mainly massive damage and failing three death saves [...]
That and the answers there seem to work under the assumption that massive damage does apply to monsters.
However the second question's currently most upvoted answer (36 upvotes) states the following:
The rule for dealing damage in excess of your total hit points is intended for player characters, not monsters [...]
These two seem to be in direct opposition and I'm unsure then what the correct ruling is.
Here are the passages I believe are relevant to answering this question:
Most DMs have a monster die the instant it drops to 0 hit points, rather than having it fall unconscious and make death saving throws [...]
Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum [...]
Does the rule on instant death from massive damage apply to monsters?