In short: was Monostatos Mozart's revenge upon the person who was probably the only man of colour he encountered within his own circles as a young man - someone happier and more successful than he was, someone of whom he had reason to be jealous at one of the most terrible times of his life? Namely, the Chevalier de Saint-Georges? Here's my theory and the reasoning behind it in the Independent. (Incidentally, this could put a slightly interesting slant on the Queen of the Night, too.)
First, here's Covent Garden's solution to the Monostatos problem. We find many remedies for that in the opera world - but little explanation of why they might have been there in the first place.