Depends on your router and whatever else the phone is doing (processing is more intense when mirroring wirelessly), but usually the quality doesn't get affected, it's the frames that do. You'll most likely see a few dropped frames, or if your phone is doing other processes or your network is too congested/not optimized, then you'll see a lot of dropped frames, in which case the image will drop down to a lower quality as the devices try to alleviate congestion by sending smaller data packets (less data, less quality, more chances of getting through in congested networks).