In The Tempest, Miranda is the daughter of the magician, Prospero. She lives on an enchanted island, protected from all the evils of the world (which in this context may stand for side effects and other imperative features).
In Act 5, Scene 1 of the play Miranda makes a speech containing the words (later made famous by Aldous Huxley as the title of his novel about the future): "O Brave New World!"
The idea is that the Miranda language is an introduction to the Brave New World of functional programming.
Because it is a proper name, not an acronym, only the first letter of Miranda is capitalised.
Miranda is also a moon of Uranus, but that has nothing whatever to do with the name of the programming language.