Fridge Logic: True, the owners of the farm (who are almost never seen) are portrayed as dense, but how could they not notice that the barn has 144 cats in it?!
For that matter, it might behoove Myrtle to do an updated inventory of the barn to see if there are now more than 144 cats in the barn. Odds are a good number of them haven't been spayed or neutered, and when you have that many fertile cats in one place, babies will happen. (No kitten births have been seen in the strip, however).
We have, however, seen plenty of kittens: first being shown the dangers of catnip, then playing with the bunny kits and April's fox kits.