Are the phrases "people are dumb" and "mean as hell" types of figurative language from In Cold Blood and are they adequate?

Metaphors and similes both compare two unlike things. 
However, metaphors use words such as, “as”, “was”, or “were”.  Similes compare the two
unlike items using the words, “like”, or “as”.

Therefore,
“she is mean” is not a metaphor.  However, “she is as mean as a snake” would be a
simile.  “She is a snake,” would be a metaphor.

These are
only two examples of figurative language.  There are other forms of figurative language,
such as hyperbole (extreme exaggeration), allusions (reference to something well-known),
and alliteration.

Best of luck!