By YouTube influencers do you mean 'social media influencers', ie people with established audiences who tend to direct and inspire conversations in the public sphere on a particular topic? If so, I would say they are inherently leaders, because it's their leadership qualities (ie., persuasive, charismatic, thought-provoking, etc.) that make them influencers.
If you just mean anyone on YouTube, I would say not necessarily. There can be successful channels that aren't tied to a personality, and they're not very influential in the grand scheme of things. Then, of course, there's lots of hobbyist youtubers whose influence is limited; they might still be leaders in small circles, but they wouldn't be considered leaders or influencers in the traditional sense.
I can't think of anyone who I think an established brand, publication or expert would consider a 'social media influencer' who isn't a leader in something, even if they're not particularly constructive or inspiring.