Plato, despite his intellect (or perhaps because of it), turned out to be, as you say yourself, "naive and mistaken".
Yet Diogenes was able to see that about Plato then, as a contemporary.
And while it's true that he never sat down to write anything himself, his work has endured via written accounts of others (Diogenes was a real person, not a mythical figure).
"Dear Ladies and Gentlemen, I am here today to convince you that the Great Gatsby could not possibly be an allegory for animal beastiality. I have three reasons for this belief. First, none of the main characters in Gatsby are animals. While Daisy does have a pet dog, there is no reason to believe that Gatsby ever expresses an interest in man-on-dog relations. [insert paragraphs two and three.] In conclusion, the Great Gatsby could not possibly be an allegory for animal beastiality, and it would be simply absurd to believe otherwise."
/Agreeing with pg, intelligence can only be demonstrated by finding what is, not what isn't.
I don't know what motivates people on reddit (don't they all upvote you automatically, like the people here?), but Diogenes' criticism of Plato was not simple whining, it was criticism based on a serious philosophy (cynicism) which stands in stark contrast to Plato's.
For a better presentation of the contemporary view of philosophers like Plato, you should read Aristophane's Clouds. It's also alot deeper in meaning than a mere facile mockery of poorly understood ideas.
It's been a while since I read the Clouds. IIRC, it was a lot of scatological and Mae West style humor (i.e. "Is that a sword under your tunic, or are you happy to see me?").
Aristophanes has literary value, but he was just exposing holes in Plato's logic for comedic and theatrical value; he was not himself a serious philosopher.
Isn't that the whole problem expressed in two words?
Euclid was arguably "more serious" as a philosopher than a mathematician -- his principal contribution was to build a logical framework around others' results.
I want to start punning now -- who was more serious, Edward Teller or Richard Feynmann? -- but it's not useful. I'll simply note a certain dissonance in calling Diogenes serious, as you seem to.
Leo Strauss would say otherwise. He has a whole book about Aristophane's take on Socrates, highlighting the crucial tension between the philosopher and the rest of society - the reason Socrates was killed. According to Allan Bloom, the whole enlightenment project can be explained as an attempt to solve this problem by displacing religion with philosophy, through science.
