My player just killed a boss that was supposed to take a lot of effort in 3 turns with random bullshit. I couldn’t be more proud.
So, this is a bit of a “depending on the group & situation” thing.
Fudging HP is definitely a thing DMs (myself included) do. However, with an experienced group who can get a sense for how much HP is typical for mobs at a given level, and if the amount of damage done is quite clearly far enough that he ought to be dead, it can be hard to add HP without “showing the finger on the scales” - at which point the illusion breaks and it becomes “un-fun”.