My guess is that the number of developers making a middle class living is probably no more than 300. My gut tells me it's less than that (total guess, of course). I bet it's 150 people world wide.
If you look at the charts, it's generally filled with the same titles/publishers and few titles that jump in and out of the rankings. I look at the Mac charts more often, and you can really see some jumps on those charts. Outside the Top 300, there just isn't much money to be had. And we know the devs at the tippity top are 1 percenters. The devs jumping in and out of the 'middle portion' of the rankings I bet is a smaller group than the 1% making 94%.
If you have apps that go in and out of the rankings (like the creator of this thread) you notice that you can get a good ranking in certain categories without making a ton of money. I do believe there is a good amount of devs making nothing, and the remaining $85.8 million is certainly not divided evenly, but I'd be willing to bet that the middle class is a smaller group than the millionaire and billionaire class. At least by half. Maybe more.
I've busted in the top 10 on the MAS once or twice in a category, and didn't really make anymore money than a normal day. I've even filed bug reports when I noticed strange results in the rankings, never heard back though. I have one of best apps in that given category on the MAS. It charts from time to time, but it never sustains its ranking for long periods of time. The ten or so apps Apple picked to highlight in the front of the App Store for this category... some haven't been updated in centuries but they are always in the front...some of them are made by developers who have even abandoned the MAS and don't even have the current version of their app on the MAS. There are plenty of other apps that are better or just as good as the ones they show (made by lots of other devs).
My point is, I firmly believe if there is a middle class, it's very very small and you have to get lucky just to have a chance to make a normal living. Something about that just doesn't sound right. I think most people are fine with having to get lucky to be rich. But having to get lucky just to make 40k a year is why DT may become Prez.