Dear Tris Fray Potter,
I myself would not use it in an academic paper, unless under special circumstances such as a discussion of its origin, connotation or history.
Hoi polloi is a derogatory term for the masses, the lower classes, somewhat equivalent to calling them The Great Unwashed...
It's maybe not quite slang but perhaps opposite in meaning to the use of hoity-toity for the snobbish upper classes (or trying to imitate the upper classes).