Do today’s Greeks bare any resemblance DNA-wise of those Greeks who established Democracy?
I don’t know, DNA testing could show. In all probability, yes, they are the same people.
I think today’s Greeks are mostly of Slavic genetic origin.
You could certainly establish a DNA line to the current batch of geniuses but remember it’s pretty well documented that a major source of the original failure was inbreeding among the hoypaloy. They went nuts. The question would be whether 2500 years has been enough time to breed out the effects of the original mother/son/daughter/sister/kuzzin kissin’? I’m guessing not for the simple reason that there’s no one left there that can fix the friggin roof on the Pantheon or anything else that they used to be proud of.