I remember there was another appearance around the same time where Nacchi had an eyepatch. I think it was 24 hour tv or something like that.
If my memory isn't faulty, and Nacchi had one as well, maybe it was some kind of eye infection or something. Pinkeye, for example, is notoriously contagious. Other than that, maybe she got hit in the eye in a perf. It seems to happen frequently enough.