I'm not sure how reliable the answer on IMDB is...but it does state in the Trivia section that her middle name is Evelyn. Here: http://www.imdb.com/title/tt0096697/trivia
I've been going through a bunch of episodes to see if Evelyn was mentioned as her middle name on the show, but I haven't found anything. But then again, Bart's middle name wasn't actually mentioned on the show either (they've only said "J."), but it was on "Bart Simpson's Guide to Life" book. So maybe Maggie's middle name is on one of the many other Simpsons books out there?