A bit confusing. A round being like 3 seconds long is ... well let's just say, it is not good, that the Start button is in the upper right corner. You could add a countdown or put the button in the middle where you would aim when at rest. It does not help that the sound stays the same while fighting and while not.
The difficulty increase is a bit steep. Basically you cannot afford any ammo after some point. And if you lose some cows you are done for. You cannot recover, as a single cow makes 50 but price for cows increases by 40. Oh, of course, if you manage perfect play (and you cannot, see next paragraph), you can get to 5 or 6 stables, letting you hold out for when ammo costs that much. Maybe you can even stockpile some cheaper ammo.
Low flying saucers kidnap almost instant. In one second you still have your cow, a blink of an eye and you have game over screen. And the high flyers seem to get cover from the low flyers.
Tnt acts too slow.
My highscore is somewhere around 12. The issue was, that I could never recover from losing cows. It starts a downward spiral. In theory, less cows might be easier to defend. But you cannot position your cows. If one is to the left and the other to the right, you lose one to the low flyers and can't buy replacement.
But I think that kind of difficulty is intentional. Not really my cup of tea.
If you want to make it easier, you could make inflation not depend on number of rounds, or to a lesser degree, but on how often something was bought. Right now I could stockpile cheap items, but not save up for a new cow. (ok, the stockpiling option is not really realistic, because you lack the money...)
Your tags do not reflect the game. (And tagging indie on itch is kinda redundant. No clue why some do that.) My tag addition suggestions would be arcade, defense, short, ufo, cow, abduction. My tag removal suggestion would be endless specifically, because this is not flappy birds. Your steep inflation mechanic gives a breaking point no matter how good you play. And the others just are not very informative about the game. Maybe fast-paced is good. But there are no aliens to be seen, only ufos. While not incorrect, your game would not be expected when I search for sci-fi.
As a side note, you will find several games where you play the ufo trying to abduct cows, if you combine the tags ufo and cow ;-) (And as your game has none of these tags, people can not find your game by association of tags)