Yes, it is.
an alliteration occurs when a series of words in a row, or close together, have the same first consonant sound.
Johnson uses sarcasm in this poem as he expresses the opposite of what he means. When he says "pile on the Black Man's Burden", and gives examples of how people can make black men more miserable than they already are ("his wail with laughter drown"), he is using sarcasm. He clearly does not want people to pile on this burden and make black men's lives harder, but he is saying that people should do it to show them how ridiculous it sounds and to point out that people are already doing that.
Mortified means you are highly embarressed. People are embarressed because they fail or they are ashamed
The second sentence, the author seems to be trying to inform or warn readers about extreme climate changes and then increasing .
