4Game

So meadows are pretty light (5ish mobs), but its the starting area, so fair play there. The Black Forest ramps up a bit with six +2 only found in the dungeon (or raids).
As much as many dread the Swamps, they are likely the most "bio diverse" of all the current biomes. 2 types of Skeletons, 2 types of Drauger, Drauger Elite, blobs, ooze, leaches, surtlings, Wraiths, and the abomination puts the swamp at 11 different things you have to deal with, not including any post boss-kill spawns.
Mountains dropped to 3 new, plus 2 skel and 2 draug, and gains 3 more in their dungeon. Plains jumps back up to about 9 if you count the different types of Fullings.
Mistlands has Seekers, Seeker Soldiers, Gjall, Ticks, rabbits, and only in the dungeons, Seeker spawn. That's 5 +1, unless you count the dwarves, which add 2 more, (possibly more if you separate each mage type)

That's a sad comparison. Sure, Mistlands is new, but so were the swamps, and the only thing that was added were the abominations.
Granted, its a swarm zone, but I think something like a deadly plant or two (especially if they add more light, ala wil-o-wisp) would be a good add, and its hide could serve as the light armor of the zone. (plains still needs a light armor. Maybe a snake or giant weasel)

I'm guessing the challenge of the visibility and terrain are partial reasons for less mobs, but honestly, its makes it less interesting.
Personally, I'd be happy if the Mist in the Mistlands stuck to its borders, keeping other biomes out and Mistland's creatures in, (especially since as it stands, a functioning swarm assault from the Mistland's creatures would decimate/ infest the rest of Valheim with ease.)