Is it natural to say "Most Americans loving baseball is a stereotype"

It's grammatical, and I could see it being used naturally in some cases. But the more common and broadly preferable formulation would be "It's a stereotype that most Americans love baseball."

"Most Americans loving baseball" is used as a noun in your sentence, but that's not immediately clear as you begin reading or hearing it. The reader can't begin parsing the grammar of the sentence until they hear its end, and then they have to backtrack.

Conversely, "It's a stereotype that most Americans love baseball" prioritizes both grammatical framing and the context of what you're quoting and why.

As an alternative, even a slightly simpler noun phrase, "Americans loving baseball is a stereotype," would be slightly more clear and have fewer possible continuations.