Yes, "dead as a doornail" is a metaphor. It compares the state of absolute death to the common object of a doornail to emphasize the finality and lifelessness.
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That grass is deader than a skeleton!
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A metaphor of a skeleton is suggesting that the skeleton is the remains of something dead, therefor to have a skeleton you have the remains of something dead or gone.
"There is a skeleton here" -talking about something that has gone and will never return but is not actually a skeleton.