Yes, he is. People compare him to The Beatles--yeah, they're OK if you're into that kind of thing, but only as a band. If I recall correctly, they all pretty much suck as solo artists, with Ringo Starr being the worst one of them all when it comes to solo stuff (although McCartney is halfway decent.) The Beatles worked as a band, but alone, they're nothing special. I know a lot of people, especially Americans, like them and think they're god or something, but I just don't see it. *shrugs*
Elvis deserves some credit for being a revolutionary, though. His stuff was legitimately quality, and he gets props for being a music/dance pioneer, but if I recall rightly, he didn't write any of his own material.
So...if you compare them on merit alone (forget sales--people buy stupid sh!t all the time, and I'm not counting those numbers simply because it measures popularity and not real quality) then Michael is indeed pretty goddamn under-rated. He truly was the best, period, and he proved it throughout his life.
He outshone Elvis because, unlike him, Michael actually wrote his own songs, and did more to revolutionize dance/music/entertainment, introducing the concept of a music video, which will live on for a long time to come, if not forever. That was his idea, his vision, and others after him have taken from it, whether willingly or grudgingly.
He's outshone the Beatles in that he was always truly brilliant alone. He was a multi-talented wunderkind, and did not really need any of his brothers to shine on. As a matter of fact, he was better without them, as events prove. The Beatles, once they broke up, fell apart, because none of them had that individual spark, and their real (I guess) magic happened when they were together. So, as individual artists, they were all pretty mediocre, whereas Michael was this lone, brilliant, amazing thing.
So, as far as I'm concerned, he's got them all beat. People just don't like to admit it for some strange, stupid reason. So yes, IMO, he is very underrated.