From what I can tell it is mature enough for stack overflow to use it

I was referring more to the overall ecosystem than the core platform, which is solid. Stack Overflow uses Dapper rather than Entity Framework, and that alone makes transitioning much easier. EF on .NET Core still just isn’t as good, and the third party library support isn’t as good either. For most people, I don’t really think moving to Core is quite worth it yet.

Stackoverflow runs on .NET Core? That's news to me.

But C# is still a better language that isn't as widely used (therefore it's underrated). And that's what we're talking about.