They pimp it all of the time during Microsoft events, but I can understand not wanting to watch one of those in the last couple of years.
I'm not sure if it was a startup that MS bought early on or if they designed it from the ground up, but it started out as "Beam" and was supposed to be a simple to use, all-in-one streaming package (compared to Twitch + OBS/xSplit + plugins + streamlabs + whatever else). It seems like OK software and can be used directly from XBOX 1 consoles.
There is no app/support for PlayStation 4 and it looks like the Mixer audience is roughly 1%-2% the size of the twitch audience, so there'a a lot of effort and not much to show for it. If they had the idea when they had Xbox 360 levels of market share, though, it would likely be huge.
Either way, if Jim showed up on Mixer for a monday morning stream and got 7k people, he would be more popular than Fortnite on that platform. Someone would immediately take notice.