As we've said before, the title came from a misprint in a newspaper, and Bishop notes this on the published version of the poem. Though it was clearly informed by the word "mammoth," the order of the words is important, as is the capitalization. Both "man" and "moth" are capitalized in the poem to give them both equal weight and to create an extra layer of ambiguity. Is he a man or is he a moth? Is he a hybrid creature of some sort, or is he a man who simply behaves as a moth? We have to read through all the way to even begin speculating on the answer to these questions.
It may also be useful to note that Bishop rarely uses fancy titles for her poems. Other famous pieces of hers include "The Fish," "The Moose," "In the Waiting Room," and "Sonnet." No, this isn't the mark of a lazy poet, it's really kind of clever marketing for her work. The title is the hook that drags the reader into the poem without giving anything away. "The Man-Moth" makes us wonder what this is all about, and next thing we know, we've read through the whole poem.