Depends on the type of zombie.
To me, “mob” or “swarm” denotes a unified, if disorganised, force. Romero-style zombies aren’t even self-aware, much less aware of each other. They certainly aren’t “pack” animals, at least not until Land of the Dead when they all rally around that one cool semi-intelligent zombie dude.
“Mob” or “Swarm” might work for the 28 Days/Weeks type “hyperzombie” creatures, though in my mind they’re not really zombies, since they have a disease, and are not the reanimated dead.
I do kinda like “shamble”.
I have to say, during the Z-pocalypse, we’re not going to be arguing semantics. It’ll just be “Holyshitballs there’s a fuckload of zombies coming from that-a-way! Let’s GTFO!”