I've found looking at data from StackOverflow to be pretty useless. "Number of questions asked" could very easily be interpreted as a negative thing -- maybe there's some ideal ratio of questions asked to raw usage numbers that would be more useful. Additionally the set of developers who ask a lot of questions on SO does not extrapolate to the wider industry.
Strange that Xamarin has declined when it's actually MUCH easier to build apps and Stabler than ever before. .NET Core on the other hand is rising in use so you'd think a Xamarin would be complimentary to that approach.
The issue truly is Microsoft's step-motherly treatment of the framework, and there are better enthusiasm in third party efforts like Uno
Some trends decline will simply be because Microsoft already announced the brand rename of Xamarin in the .NET 6 timeframe to MAUI [Multi-Platform Application UI]. With .NET 6 expected to finish the .NET/Mono merger, and a bunch of new investment expected into MAUI (demos included a new "One Project Only" Visual Studio approach that excited some of my friends), there should be a lot of new enthusiasm for the projects. Just the obvious downsides of brand renames is that it affects "Trends" charts.
http://www.fluttercoder.com/
Notice the decline in Xamarin