ok, I've got these mods installed:

Dawnguard
Unofficial Patch
Skyrim Flora Overhaul
Enhanced Blood
The Dance of Death
HighResTextures
SkyMoMod
SkyMoMod_standard list
Armory of Tamriel
Crafting 300

I think random monster encounters in the wilderness is too frequent. Is there a way to fix that with a simple mod?

Wild guess here, but I think SkyMoMod is responsible for adding more encounters out of those mods.

Does the readme say anything that could help?

Yep it is Skyrim Monster Mod. It adds several creatures to lists which are supposed to spawn only non hostile ones. It uses for example the rabbit & deer list to let new creatures appear.

Yeah, I know it's SkyMoMod which is the cause, but it says nowhere that MORE monsters than normal will appear. Is there a fix to make spawns less frequent?