Quoting from the Onion A.V. Club article that coined the phrase (http://www.avclub.com/content/node/57870):

The Manic Pixie Dream Girl exists solely in the fevered imaginations of sensitive writer-directors to teach broodingly soulful young men to embrace life and its infinite mysteries and adventures. The Manic Pixie Dream Girl is an all-or-nothing-proposition. Audiences either want to marry her instantly (despite The Manic Pixie Dream Girl being, you know, a fictional character) or they want to commit grievous bodily harm against them and their immediate family.

Not everyone minds the stereotype -- some stereotypes are perfectly likable. But it's certainly evolved to the level of stereotype.