Smug mugs = mug is a euphemism for face; smug = self-satisfied. The term means self-satisfied people.
All this [does 'all this' refer to the previous paragraph, or does it include the one before the previous paragraph as well?] It refers to all the preceding points made
How much mutation [can 'mutilation' be used instead of 'mutation'?] Absolutely no. Mutation = change; Mutilation = cut up or alter radically so as to makee something imperfect.
a wilting wall flower [does 'wilting wall flower' refer to a depressed person?] It means someone without self-assurance, lacking in assertiveness.
a magical mask that Stanley Ipkiss uses in the eponymous film). Reference to this: The Mask - Wikipedia, the free encyclopedia
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