The real Cloud probably cry if he knew Zack resent him even a little.
In JP, Zack said "すこし憎たらしいけどね (I guess I hate you a little)" 憎 (Niku) is basically hate; can be translated as detest, awful, horrid, odious.
It's not even Cloud's fault that Aerith likes him, he has clueless that she likes him (till her confession in the dream date I guess). So sure, it will make him guilty even more. He has forgotten Zack and even though his memory of Zack's death is wrong, he's already concerned to tell Aerith the truth but he doesn't do in the end (idk I think Cloud should talk about it knowing Tifa doesn't tell Aerith as he ask), and now Zack 'resent' him for that.
Quite so, and this is significant in the LTD because one may note a recent trend of Clerith shippers to announce, decades down the line, they actually really love Zack. The reason for this is they believe the scene in Rebirth where Marlene jokingly tells Zack that Aerith likes Cloud, and Zack reciprocates in joking manner, is somehow an
acceptance of this fact on Zack's part - vis, he instantly moves on from the girl he died trying to get back to, while giving his blessing to his friend that, had he not saved, he would have arrived back to Aerith - based on the words of a 5 year old giggly girl he only just met.
Now, anyone who genuinely believes that this is the case has clearly never played Crisis Core, nor has any objective idea who Zack is or the nature of his relationship with Aerith, but I'd also argue it requires a considerable suspension of belief to reach this conclusion under any circumstance.
Let's leave aside that Marlene and Zack's conversation is done teasingly and is stereotypical of the sort of thing girls of that age do. I think it's safe to conclude she spoke the truth regardless: Aerith likes Cloud. She says so herself. Her entire arc culminates in her realizing she
does like Cloud but doesn't quite
like-like him; after two whole games of continually forcing him to be the Zack she's yearned for ever since the real thing abruptly exited her life. Again, as the devs have now repeated for the slower among us, there are various different levels of relationship and liking at play in the games.
The Gold Saucer dates air these differences very openly. Cloud himself loves each and every person on said dates but only one of them does he pull in for a kiss. That's not to minimize the feelings he has for the others, but they are not the romantic love he has for Tifa alone. It's not any less, it's just...different.
Nevertheless, the Cleriths will read Aerith as liking Cloud even when the game quite clearly says otherwise, so too then they assume Zack has accepted this. I think this scene illustrates that not only does he not accept this scenario, but Cloud too will carry those words until his memory is returned and
also will not accept this scenario.
There's plenty of evidence to say that it won't come to that, however. We know Aerith still loves Zack, she tells Cloud - point blank, to his face - that she does. She asks about him constantly, thinks about him unendingly, always finds her way to him somehow, directly or indirectly. Gongaga again presents the perfect example, while the party all scatter Aerith INSTANTLY manages to find Zack's parents. It's not by chance, she was looking for them, she was looking for
him.
Ironically a popular CA talking point is that in AC we see Cloud "searching for Aerith", a dressed up way of saying he's ignoring his family and looking to die, but the real searching has come only from ZA. Zack hunts for Aerith across dimensions even, Aerith searches longingly for him through Cloud, continually finding herself unable to get beyond the parallels in their actions and heart.
And here's the rub: Zack won't give up that pursuit. Even in Maiden Who Travels The Planet, glorified CA fanfiction at best, he never stops trying to win back Aerith's heart. No matter what will happen, he will succeed because she wants him to. This is the pay off we'll get in part 3 that the devs have been building up to in Rebirth, the happy ending for ZA and CT that these characters have fought for all these years. Who can say it's not richly deserved?