The "dumb" part is that neither the graph nor the quote are his. He just slapped the two together and it made a hit. He brought little value to the joke.
But that's how memes work, they mutate, evolve, and what he did was he introduced a slight mutation in the original joke that helped its spread. Maybe his Twitter account also has the right connections.
I think "dumb" in text form might be the wrong word. In conversation this would sound normal to me. Given the blog post of what he is talking about, "dumbest" sounds better than "my super simple joke I thought of and made in a few minutes".
Ironically, I think he partly considers it "dumbest" because he didn't overthink it... like I'm doing to the author's use of "dumbest" hehe.
But that's how memes work, they mutate, evolve, and what he did was he introduced a slight mutation in the original joke that helped its spread. Maybe his Twitter account also has the right connections.