Anyone in the public eye - singers, dancers, writers, actors - have their good performances and bad. No one is on the mark all the time. You have to look at a body of work in order to judge it fairly. Comedians have it tougher than dancers, writers or actors in my opinion. This is especially true if their work is based on current events, political commentary or modern life in general.
Dancers and actors can impress audiences many years after their films were made. Writers do the same with their books. Comedians less so.
The era into which you were born plays a big part as well. George Carlin's routine about seven words you could not say on TV was more a social commentary than comedy, but his ability to tell the tale made it work. Much of his work will become less understood with time. The reason? Most comedians are funny because they make you laugh in the now. Whether it's using a telephone to find out about to-bac-co or making light of current events like THH22M does, comedy loses something over time. How we think, how we perceive our world and even our temperament change. Our language and technology evolve.
Lenny Bruce was considered the first to use strong language mixed with edgy political commentary to make people laugh. It worked then, but is nothing special today. The trick is being able to recognize that he was a pioneer in the field. The same goes for Seinfeld, Carlin and others. They were a part of the moment, but left an impression.
Some people were not born when marijuana was in vogue (yet again) in the late 1960s and 1970s. To listen to a comedian describing Richard Nixon, Lester Pearson or Robert Stanfield sucking on a doobie, then messing up a political speech worked then - but not now. These men are dead, relatively unknown to younger audiences and the "newness" of drugs has passed. Times change. Artists come and go.
Some performances are truly special, but they are a minority. Only the extraordinary ones transcend time.