If by "shitty" you mean not exactly technically gifted? Plenty mtb youtubers do market their channels as mountainbiking from the perspective of the average/mediocre rider as I assume it gets them views from other average/mediocre riders. and let's face it. That's who makes up the the vast majority of mountainbiking. I don't think many make a full time living out of it. But if it pays their expenses or they get a few freebies and they genuinely enjoy filming and editting themselves fair play to them. Personally I find watching shite/mediocre riding pretty tiresome. Even more so having to listen to their every thought as they do so and suspect you do too.I'm sure there are a few out there literally marketing themselves as shitty riders.
Then there's the UK Youtube Legend Dave Jenvey. I'm not sure if he even thought about marketting... but this guy is both difficult to watch and at the same time the longer you do the more difficult it becomes to break away from rubber necking.