How to fix stripped tire valve stem?

To me it looks more like it's not round anymore (taken a beating). In your case I don't think there's much of an option besides getting a new one installed. You could try to put a die on it of the right size and get the threads worked out ... you'd be in no worse shape.

As an aside, while not optimal, if you're not leaking air, you don't really need to worry about it too much. If it isn't, I'd suggest getting it replaced at the next tire change.

You could get a flexible vinyl cap, like an automotive vacuum cap . Or the hardware store sells vinyl caps to cover exposed threads on gate hardware, etc.