OK. I love the Fourth World, they're brilliant comics, but you have to understand that the context was WILDLY different in terms of what Kirby was doing. Basically, being as ahead of his time as he was, Kirby had been reading about how superhero comics were "modern mythology", and he decided to make the Fouth World to explore that concept in depth. But he was also seeing it in the context of the time, in which superhero comics were big, bombastic and ridiculous, and--and this is the crucial part--pop culture was a lot sillier, campier, and more unrestrained. Kirby was making these comics at the same time that David Bowie was breaking through by pretending to be a space alien fashion victim, and movies like Barbarella and Danger: Diabolik were the cutting edge of cult cinema, to say nothing of what comics of the time were like. That said, this was riiiight around the time superheroes were starting to style themselves as more SERIOUS and RELEVANT, with absurd stuff like Green Arrow/Green Lantern and slightly less absurd stuff like Harry Osborn becoming an addict in Spider-man. That's the path superhero comics took going forward, but Kirby was playing in a different sandbox. Rather than trying to make superheroes realistic, he went with the Joseph Campbell idea that there was something deeper and more mythic lurking behind these childish stories. So on the surface, he didn't steer clear of the childishness and campiness and imagination; in fact, he embraced it. The characters are literally gods, after all. They would embrace their function in their names. It's the deeper themes and ideas he tried to invest with meaning, and did so a great deal more effectively, to my mind, than most other superhero writers until Alan Moore came along.
The original, Kirby-penned Fourth World makes absolutely no pretense at being "realistic" or even "naturalistic". We've gotten used to this in pop culture, the idea that genre movies and TV are always going to stoop to meet us so that we take it seriously. Because heaven forfend any of it seem childish or ridiculous. But Kirby's Fourth World exists unapologetically as a Silver Age comic book, with often hilariously cheesy dialogue, over-the-top characterizations, sweepingly imaginative imagery, and an embrace of weirdness. It works because it's uncompromising. The later attempts to use Darkseid (who admittedly sort of works as a character on his own, even though most other comic writers don't really get the multiple levels Kirby was trying to invest him with and just make him a Super-Big-Bad-Villain) come off as if someone was trying to make a "realistic" Season 3 of Twin Peaks, or a Muppet movie where Kermit was played by a realistic CGI frog. It's missing the point, even aside from being divorced from the original creator's unique sensibility. When you try to drag Darkseid, or any of the other 4th World characters, into the DCU proper and make them firt that sensibility, it often doesn't work. They have to exist in their own skewed reality; take them a few steps further towards "realism" and they fall straight into the uncanny valley.
That said, as Chris Sims pointed out in a column a week or two ago, Darkseid is definitely bound up with the DCU, regardless of what Kirby had intended (he wanted the 4th World to be his own standalone project originally). It's just important to remember that Kirby's Darkseid is different from everyone else's Darkseid.