Nice entry! I don't usually play strategy/tower defence games, but hey, for the stream am I right? xD
I like the simpleness of the graphics and the people. It does seem to work well! Once thing that would make the game better is adding the level of each upgrade, because there is no way of telling what you should upgrade next. Maybe also make the upgrades more expensive after each level upgraded and add some kind of higher material reward for killing stronger zombies (with some visual changes to differentiate them)
Great job overall!