In my view, yes it's worth it, especially if you are a junior developer.
While it's not ideal, many many .NET projects will be .NET Framework for a while yet. And it's really not a whole heap different when it boils down to it. If you're a graduate/junior, most of your time will be spent getting to grips with C#, and things like Entity Framework, Azure, Cloud computing in general, front end development techniques etc. And so the small differences between .NET Framework and .NET Core won't be that much of a worry at this stage.
In the future, of course you should be looking to get your hands on newer things, but I just don't think it's going to be that much of a hurdle for future work opportunities.
If you get stuck doing VB on the other hand...