There is no quality loss during import but make sure when you share your project that the resolution is the same as that of the imported clips. iMovie sets the project resolution from the first clip placed in the timeline. The default seems to be 720p so if your clips are 1080p make sure that you put a known 1080p clip as the first clip added to the project else it may be set to 720p (you can remove this clip later).
If you are referring to the quality of the video during editing then this may well look inferior since iMovie is having to decompress the highly compressed AVCHD video 'on the fly'. How well it does this depends on the speed and resources available on your computer but it won't affect the qulity after sharing.
Geoff.