Pink Diamond is an asshole and her reasons to be with the humans feels questionble.

Zim at least has done an few heroic acts like Helping Squidman find his family

Stopped Planet Jackers from Destroying Planets

Defeated Sergeant Slab Rankle Zombies

Zim is arguably far less sympathetic than Pink as he engages in horrific atrocities with sadistic relish, to blatantly attempting mass scale genocide on the human race with the added bonus of evil ass experiments he done on humans. Don't forget the mass destruction and death he did to his own people during Operation Impeding Doom 1 which he showed zero regrets for. Pink at least cares for her family and the human race and has much more heroic moments than Zim, who's egoistical psychopath and a narcissist who rarely cares for anyone besides his own self-declared glory. Hell, him saving the Earth from the Planet Jackers is solely for the fact he wants to conquer the Earth and thinking they're stealing what is basically property to him and his kind. He has zero issues destroying the Earth himself as he shown on several occasions.

Zim is far from sympathetic as you can get. He's a sadistic psychopath who is funny, but Zim and Sympathetic do not mix at all in my opinion.

EDIT: Bruh I can't believe I accidentally voted on Zim when I should've voted on Pink Diamond-

That still doesn't make Pink Diamond more "sympathetic." She is worse than him like she abandon her own follower in a fucking planet for many years until she turned insane.

And Zim has no morals whatsoever ever, he do that to plenty of other people for the sake of his own ego, Punk at least has plenty of redeeming qualities to her while Zim barely has any beyond a small few pet the dog moments and yet still a massive dick and a sadist 99% of the time. Again idk what sympathy Zim even has compared to Pink.

I'm sorry but if you actually think Zim is more sympathetic than Pink Diamond then there is some straight-up DILP going on. You want to talk about leaving your own follower on a planet until they go insane? Zim does genocide. In the comics, Zim kidnapped children and had a machine that could erase them from the universe. All of which is worse than anything Pink Diamond did.

Holy crap, that's evil as hell! Yeah Pink at least cares for the human race, she has some redeeming qualities and genuinely does mean well. Saying Zim is more sympathetic than Pink is borderline DILPing and forgetting all the depraved things he has done with none of the redeeming moments Pink has. Hell he'll do the same thing she does but ten times worse.