What is it called when you replace a noun with another appropriate, but humourous noun to get your point across?

ex. Marissa, as sweet as you are, your meaner than a salt shaker.

ex. Taylor, your such a zit.

Is it an oxymoron?

Thank you for using the Jiskha Homework Help Forum. Here is a definition of "oxymoron."

Main Entry: ox·y·mo·ron
Pronunciation: "äk-si-'mOr-"än, -'mor-
Function: noun
Inflected Form(s): plural ox·y·mo·ra /-'mOr-&, -'mor-/
Etymology: Late Greek oxymOron, from neuter of oxymOros pointedly foolish, from Greek oxys sharp, keen + mOros foolish
Date: 1657
: a combination of contradictory or incongruous words (as cruel kindness)
- ox·y·mo·ron·ic /-m&-'rä-nik, -mo-/ adjective
- ox·y·mo·ron·i·cal·ly /-ni-k(&-)lE/ adverb

Now, what is your opinion?