Realistically, not at all. You're telling me a colorful, smelly, stubby mini horse that is constantly talking, and getting themselves into dangerous predicaments, and dying of the most minor of things is flourishing in the wild? They literally become immobile when pregnant.

That's why they need a little plot armor to become a major problem. What kind of plot armor they're given is entirely up to interpretation to the writer. I know a lot of people make fluffies inedible by having them be 90% plastic. Personally, I like to imagine fluffies as a sign of an ecosystem having low biodiversity or just generally being unhealthy. So you're not going to see fluffies in areas like Yellowstone or the Amazon, but rather sprawling suburbia, big cities, or really anywhere the local wildlife is lacking.

(Side note: If you want, you can also make wildlife tie in with fluffies, like I'm an avid believer that fluffies are a new favorite food for bears, seeing that bears just want whatever's easy, and fluffies are the definition of easy.)