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The highest res on the clip is 360p - standard def camera? Also, what version of iMovie and how are you going from the finished iMovie project to YouTube?
 
iMovie uses subpar codecs. The quality of the codec will make the most difference for youtube. You could also forcefully get more bitrate out of youtube by manually upscaling your video to 1280x720 or 1920x1080.

Youtube gives 800kbps for 480p, 2000kbps for 720p and 3500kbps for 1080p.

Best codecs to use for youtube's lower bitrates are x264/h264, which are not that well implemented in iMovie.
 
My camera is a JVC Everio HDD

I've used that camera so it should be pretty good quality. In iMovie click Share then Export Movie... I would pick the best quality setting available and upload the resulting .mov file to YouTube. If you want to customize the export format and settings then click Share and Export using QuickTime you will find additional options. For most purposes the standard Export feature works very well I use it for 1080p all the time and it's great. If you have other questions send me a message.
 
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