I came into this thread thinking that he wasn't, but I've changed my mind.
In my opinion, anybody who can write as many memorable, timeless, catchy, elegant, pleasing melodies and hooks as he did is a genius. He is a pop music genius. His shit songs would be amazing for other people. I'm not a stan--my love for him isn't in the same galaxy as my love for Janet--but I always enjoy hearing him in my headphones. I like all of his albums. His singles are mind-numbing in their sheer excellence. There have been very few people to be as consistent as he was, ever. He didn't grow very much, but that's not necessary for genius. The greatest of the greats usually fell off after their peaks too... nobody can get better forever. (See Prince.) His genius is in itself his pop sensibility.
That extends to his vocals too. The way he wrapped his voice around a song was beautiful.
Edit for adds. I was going to add something before I read up and noticed that Austin basically hit it. What I'M saying is that there is no measure of genius. That was the point in writing that growth isn't necessary for musical genius. In music, you can be a genius in many ways. You don't have to be all-encompassing.