It would be unkind to single anyone out so I am not going to. The thing is, I can for the life of me not think of a single photo influencer/youtuber who is also a popular _photographer_. They're popular influencers/youtubers.
As for what I mean by mediocre: let's say you are looking at a portrait of someone you don't know. If you can't remember it 10-30 minutes later, it was probably mediocre or worse. Would you recognize the subject if you met them on the a street one day later?
Most portraits tend to be bad because they completely fail to capture the subject. People fuss over lighting and editing and color grading and whatnot, but they don't actually pay attention to the person they are shooting. I see quite a few of these people with huge social media followings who can't, for the life of them, take pictures of humans. And yet, they teach their inability to make portraits to others.
I also know professional photographers who are genuinely bad at taking portraits. And then there are those rare people who just nail it most of the time. Notice this when looking at a portrait of someone you know. Is it "them"?
Another category where you see a lot of bad photos is wildlife photography. You will see endless pictures of birds that possibly could go in a bird-spotting book purely for identification purposes. But, to steal a line from my wife after looking at a certain facebook group "it's just a bunch of tack sharp ducks set against blurred out sky". And my wife spends an inordinate amount of time looking at birds.
All you need to make bad nature photography is a big lens, a location and some time. It takes no talent. All you need to make technically good, but completely pointless nature photos is a big lens, a location, time and a decent modern camera. Then turn on 3D tracking and spray whenever something moves. Animals live in nature -- they belong in context -- they do things. Good nature photographers manage to communicate this.
(I was actually tempted to name the "it's just a bunch of tack sharp ducks..."-group, but I'm not going to. Though it isn't that hard to guess).
We drown in technically excellent images that are dull as crap.
(To be clear: I'm a mediocre photographer. I'm very aware of it. I occasionally shoot something that may be worth looking at -- but still rarely something you'd remember)