Might be... but (I assume; I don't have any specific knowledge) that Stadia runs everything on a single known software stack, and quite possibly a single hardware stack. This would make it an easier platform to target than the wider range of general consumer hardware running Linux.
Personally, I run Linux as my daily driver and I hope game developers see the success of Steam's Linux support and decide that targeting Linux is viable and worth doing. But I wouldn't assume that building for Stadia necessarily has a big overlap with building for consumer Linux.
Consider that Stadia is not the only cloud gaming platform. Also, even if the overlap between Stadia and consumer Linux is small, it's still probably much bigger than the overlap between Windows and consumer Linux
Personally, I run Linux as my daily driver and I hope game developers see the success of Steam's Linux support and decide that targeting Linux is viable and worth doing. But I wouldn't assume that building for Stadia necessarily has a big overlap with building for consumer Linux.