I'm a sucker for zombie games, so of course I had to play yours:
This was the furthest I got before I couldn't find anymore soldiers to devour.
That first moment when the camera zooms out when your horde grows was awesome. Although small I thought it added to the experience. Being able to see more of that throughout the game would be pretty great, and add to the sense of your growing army.
The soldiers taking out some of your zombies seems to be the challenge here (besides also finding the soldiers, which I think is the bigger challenge). If the soldiers can react faster to start shooting, or increase their fire rate, this would add more to the challenge, I think for the better. This would also help emphasis the obstacle of dealing with clusters of soldiers, which I saw a few times in your game. Those clusters should be even more of a worry for the player, and make me second guess myself if my horde is big enough to take them on. Right now, given the late reaction and fire rate, it's pretty easy to take on any sized cluster.
I like the scenery you added to the map. I think adding more would really help when it comes to exploring and finding the soldiers. Also, at one point I did get lost in the map ( I think the top right corner) where no scenery was present, so I couldn't tell if I was still moving up or if I've hit an invisible boundary. Besides additional scenery, adding something that clearly marks the boundaries would also be helpful and make the game more fun.
Finally, I can't tell if the number of soldiers available on the map matches the quantity of brains needed. For example, in the screenshot above I have 4 brains left. Are there only four soldiers left, or are there a lot more soldiers available beyond 4? Given the map size of the game, ideally there would be more soldiers present on the map than the target brains to consume, to give players a chance to finding most of the soldiers (and not necessarily all of them). Either that, or consider making the map smaller to make exploring more viable, or adding more landmarks to get a better sense of navigation.
At some point, the horde shown visually wouldn't grow anymore. I would be able to pick up more zombies but the shape and number of zombies shown in my horde wouldn't change. Nothing game breaking since mechanically I think it was fine, but was definitely noticable.
Great job on your first game jam! I thought it was great playing this ever-increasing horde of zombies to eat more brains!