It doesn't look so bad in the screenshot, but for me, there is a metric-shit-ton of mobs spawned SOMEWHERE on the map. What concerns me is that has gotta be more than the vanilla mob limit of 70, but most concerning is where the hell are they?
I switched into creative mode and flew all the way up to Y: 256, and the mobs weren't there. I fly down into the void, and the mobs aren't there, and they aren't inbetween either!
I honestly can't think of any mods that would allow so many mobs to spawn like that, and especially not in some wierd Y value that isn't 0-256.
*edit* They are moving around, so it isn't a bug. But I turned on peaceful mode and they're still there