Well, this is the newest thread on the subject of mobs, and the sticky says avoid making new topics. Apparently necromancy is encouraged here; I find that unusual but not otherwise a problem.
Maybe this is best done one mob at a time:
Zombies. A re-skin would be nice, but not necessary. A moon full of dead people is creepy. Why can't you get away from them? Why are they always around? Who lived on the Moon and Mars before it was uninhabitable? Why are they undead now? All excellent stuff. Just remove the O2 gear, it's ridiculous. Leave them taller if you want to show off "low gravity stretching" and have them be unique, but the helmet & tanks were a silly idea. At the very least make them look broken.
Skeletons. Same as above, really. I'm on the fence about the dual-wielding bows thing. It's cool, but just slightly too weird to suspend disbelief. Consider something different, like those throwable meteor chunks or something based on them.
Creepers. Are implied to be plants, so the O2 gear should go, and they shouldn't appear on the Moon at all (or anyplace with no atmosphere). Make a connection between Martian creepers and the vines or slime and other plant life you've put there (maybe Mars gets evolved Wither skeletons and Creepers become the ranged mob, shooting those poison vines at you). The Moon can get something else that blows up randomly, perhaps a meteor-monster (how about that for a jerk move - 1 in 20 meteors is a monster egg that hatches something like an explosive rock monster?)
Spiders. Can't breathe through their heads, so O2 gear is doubly silly. I'd straight-up replace these with something different but fast on both the Moon and Mars. Capitalize on how high things can jump on the moon - create a mob with some better control over its jumps, something that can very easily catch a running, jumping player and get it into melee. Some kind of worm or hopping insect for the Moon, some kind of spiky scorpion thing for Mars (that one can just be fast, like spiders).
Golems. Shouldn't suffocate, ever. Snow Golems should do OK on Mars, but probably would sublime into vacuum on the Moon as if it was a hot biome.
Slimelings. If you're going to have them breed in RGB, then there should be a Green egg, not a Yellow one. Ridiculous. Some more logic, slightly less randomness in their favorite food would make them feel more consistent, or else go the other way and greatly expand the list of things they can eat. Expensive food should be more effective (gold, diamonds, lapis, & redstone, probably). There should be live slimelings on Mars somewhere, and some logic to their egg placement would be fun (of course there'd have to be surface features other than "more dunes" for there to be any logic to placing anything on the surface...)
Sludgelings. Things that come in swarms this big should never have more than 1 heart. Their attack should not create any knockback. They're already a damned-near-death-sentence in their environment, they're an almost perfect storm of "you-cannot-POSSIBLY-get-out-alive-no-matter-what". They should also drop something (preferably Desh, about as rarely, or even more rarely, than Silverfish drop silver nuggets - maybe 1/9th-ingot Desh nuggets of that isn't already a thing. Desh dust? Something).
Vines. Eh, I see these were a trap to go with the Sludgelings, OK. But why not give us a reason to grow them? Maybe we have to use a modified Terraformer to make a grow chamber with Martian atmosphere to grow them anywhere but Mars? Maybe we can harvest them for superfood? Food other than apples from Moon villages would be a great thing, but I'd see having to process it heavily in this case.
Passive Overworld Mobs. I get that the Slimeling was a wolf replacement since you can't take them with you... but you deliberately built the dungeons so pets aren't helpful, and the caves even more so - they'll just spawn more Sludgelings and get killed immediately. So I don't see a need for O2 gear for them... but a means of setting up a ranch would be useful. You still have to keep the sheep or whatever in a pressurized environment, but some kind of container lets you ship them in your rocket...
For the future:
Asteroids. I don't have GC 3.0, but if there isn't a fair amount of Enderman shenanigans going on there, I'd be disappointed. The End already looks like an asteroid; they should be connected to this place.
Jupiter / Saturn. There had better be something cribbed off the Ghast as a floater-alien in the gas giants.