No Mob Spawning On Plants

Cows on mushrooms

In the Mushroom Island biome, cows often spawn on large mushrooms

Mobs inside the leaves

In the Jungle Forest biome, various mobs can spawn inside trees or leaves

Mobs on tree leaves

In the Jungle Forest biome, various mobs can spawn on trees or leaves

More various mobs

Twilight Forest mobs and vanilla wolves in the Deep Mushroom Forest biome

Wild Deer in leaves and on mushrooms

Wild Boars on the tree and in the leaves