great job for your first jam !
missing a lot of things to be honest ...try adding screen shake, sfxs ( shoots , move , zombies ) , simple music looping in the background , button show some change ( like scale up ) when got hovered .. all of that make the game feel more alive , and the beauty in this ? adding those things don't take a lot of time but make it 10 times better.
also try making the main gameplay loop inspired by the theme not just the story .. that will also improve your rating.
please don't take my words as criticism ... Iam saying things I have learnt through my journey and my only goal is to help.
and as I have said for your first game jam that is an amazing work !!! , good luck in your next jam