I would like to add some of my own observations here. I'm a short, pudgy, grey haired, 56 year old, pasty white guy with a bad complexion. This has been attested to by one of my viewers who pointed out that my videos would be much better if my face weren't in them. In spite of these handicaps I Have 850 subscribers and 920,000 views. Fortunately, I'm only in about 40% of my videos. This is possibly what has saved my channel so far. But, I have noticed that I have a higher retention rate and interest in videos that I am in. I think this may be because people like to feel they can connect with the person in the video in some way regardless of looks. In my case I think in some way they connect by voyeristicly experiencing my life.
I'll be the first to admit that I have a small following of core viewers that are probably as aesthetically challenged as I am but, the odd thing is most of my comments and personnel messages come from people that are much younger than myself. I think what can be taken from this is that you can have a successful channel and not be in the top percentile of physically attractive youtubers. Yes, a pretty face will give you an advantage as far as its being an attractor to get people to click on your video but, so is an interesting thumbnail or catchy title. Once your potential viewer is tempted to click view, you've still got to hook them with good content. A pretty face will not save bad content.