His interactions with Cloud post-time skip generally have him being the more optimistic one. Granted, Zack is certainly more serious post time skip, but it's kind of understandable, but I wouldn't describe him as mopey though. He gets a bit moody when Genesis turned out to still be alive, but I feel even the player would find that relatable since we were all sick of dealing with his whiny ass. I mean maybe Zack is moepy, but frankly, he's a big ol bag of sunshine compared to the rest of the cast who wear grim like they were all wannabe goth kids.