If you only look at the greatest Greek heroes, yes, of course their offspring don't surpass them. They are the greatest heroes, after all. I mean, Telemachus is no slouch, but you're comparing him to Odysseus.
The greatest heroes do have heroic fathers as well:
Peleus was king of the Myrmidons, a member of the Argonauts, and participant in the hunt for ...
There is a list of offspring of Zeus on Wikipedia. To avoid simply repeating it, I've tried to add more detail. This list is not complete, as they are many "possible offspring", and many with confused or unclear heritage, which may be Zeus', but may not.
Perseus, son of Danae, who went on to behead Medusa and save Andromeda. Conceived when Zeus was in the ...
Achilles son of Peleus.
Peleus was a minor hero and companion of Hercules in various of his adventures, and was also with Jason as an Argonaut.
His son Achilles was more famous as the leading warrior of the Trojan War.