Don't limit your search to the US. The "dictionary word" problem is reduced in worldwide search, which shows a more convincing trend for flutter. Though I wouldn't say it has a large or clear-cut lead on React Native.
https://trends.google.com/trends/explore?date=all&q=react%20...
How did you query this? I want to have a look, but the fact "flutter" shows data since 2004 means the terms I'm using are inflated [1]. Flutter is a dictionary word after all.
[1]: https://trends.google.com/trends/explore?date=all&geo=US&q=r...