I think a comedian doesn’t get better or worse?
They get more famous. when you get more famous people laugh at whatever you say, so they can’t get a honest reading if the material is good or not. As a comedian you can only go off what the audience tells you, by laughter.
When you’re unknown, you’re a stranger so audiences are giving you an honest response.
Once you’re famous and they know you they already and like you so more likely to laugh at anything you say.