Hurrah for the first game!
I know you downplayed the art, but the cow sprites are adorable.
One mechanic you could try is the cows getting fatter from eating the grass, and therefore being worth more points when you abduct them. That adds a bit of risk/reward to the game as players would avoid catching the cows as quickly as possible.
BTW, I used Godot and had the same screen size issue when exporting to web. Changing 'Canvas Resize Policy' to 'Adaptive' under the HTML export settings fixed it for me