I suppose looking at javascript sources and whitelisting by domain isn't impossible with the UI tools available in Qute but I don't think anything like that currently exists (I've looked around a little).

Still a real ad-blocker would be great, youtube is barely usable these days with three layers of unskippable ads on each video. (although you can work around that /specific/ problem with umpv and a hotkey to launch it: section10 https://qutebrowser.org/FAQ.html )