academic job offers during Coronavirus
Yes, it is appropriate to explore the situation. I doubt that the offer would be rescinded, but who can say except them. They will need the personnel now as much or more than ever. 

You have a signed contract so they are bound as well in all but the most extreme situations. 

But timing and such might be disrupted. And if you aren't already in the US, note that border crossing is very difficult if even possible. 

But ask, and ask to be kept informed.