panic is a strong word. I think there are bound to be a few semi-histrical people in a country of 1.3B. In any case, people have been wearing masks outside for years before any nuke incident and taking responsability for yourself before expecting the government to is probably a good rule in China as well as anywhere else.

Of course, people are focusing on the wrong risks. Walking in moving traffic is probably worse for your health than a possible radioactive cloud, but of course that's not what the media's highlighting.