I have finally found a mushroom biome, and I feel like sharing it with the very, very few people who read this. I took some screenshots of the Mooshrooms and the landscape.
A mooshroom defending its home mushroom
A mooshroom calf and its mother taking a swim
The vast rolling hills of the mushroom biome, made of mycelium
I have a tip: The red mushrooms are cramped and hard to make homes out of. Instead, try making a home on top of a brown one. They're bigger.