Greetings! Concerning the Catapult spell, you can magically launch an object of your choice (rocks, skulls, vials, etc.) that weigh between 1-5lbs up to 90ft in a straight line, but can you also launch small creatures? Say a Frog, Rat or Quipper? No.
— Jeremy Crawford (@JeremyECrawford) March 7, 2018
Well, it specifically says “object”, so no – creatures don’t count.
Presumably, they purposefully wrote it this way because don’t like the idea of you using small animals as projectile weapons. That said, if you kill a creature, it leaves behind a corpse which by all definition is now an object. If you want to chuck dead rats at people for some twisted reason, technically you can.