Hi YSSPark! I'm having the same quality issue via iMovie to YouTube. I have ipad2, which is terrible camera quality to begin with, but my regular craft videos show up fine on YouTube when sent directly there via Capture app without editing. Not Great, but viewable and enjoyable. Not so when edited with the iMovie app, the quality is so poor that there is no point in having paid $4.99 and spent hours editing to not really be able to see clearly what I'm doing in my craft videos. Even the photos taken with iphone 5S, which is better quality camera than ipad, look grainy in the montage portion on my video.
Anyhoo...I was checking in ios Settings to see what versions of each app I have and latest ios, to give accurate info. here and I noticed when I clicked on iMovie in settings, there is a selection you can make to choose, Record iFrame 720p. Perhaps this will help you. I've turned in on in my ipad. Because uploading in the best quality available from iMovie to iPhoto did Not change the poor quality of my edited video. I may need to go check the settings in the Capture app as well, sometimes when they update it gets kicked back to low quality for faster uploading.
Hope this helps, since no one has noticed your question.
Paulette
ipad2
ios 8.4.1
Capture 2.0.2
iMovie 2.1.2 (1845-8)