To some degree, it’s all a matter of verisimilitude. I don’t know that the Surfer is any less silly than the Black Racer when you come right down to it. But given his context, the story that introduced him and the way he developed, and the way he was characterized, readers were able to move on past the silliness and simply accept it as a part of the character. Whereas the Black Racer never quite got that same amount of screen time or personality (at least in part because his human form was paralyzed and couldn’t speak), and so all people had to focus in on were the surface elements, including the silliness.

And I think that if Jack hadn’t left Marvel, the New Gods characters would have been completely different. By all accounts, while Kirby had all of those designs done (and a few more besides that he never got around to using), he hadn’t completely landed on who or what any of them were. It wasn’t until that DC deal that he sat down and committed to one approach for them. So, had he stayed at DC, Lightray might have become a foe of the Fantastic Four, Kanto might have been an ally of Thor, and Mister Miracle might have battled Captain America. It’s impossible to know.