They have to be funny and varied. If there’s only one funny thing they do, that’s boring. They should also be unique. The appeal should be fairly immediate - I don’t want to have to watch dozens of videos and read a novel’s worth of tweets to understand why they’re funny. And they should produce content fairly frequently.
They should be kind of an asshole, because laughing at someone who deserves it is more satisfying. But they should also not succeed - they should suffer consequences of their assholery. It’s a delicate balance, because they should also not learn from their mistakes.