Make the cupcake enemies into rare monsters... Have them drop the temporary cupcake helmets uncommonly (15% perhaps?) and confetti pills the rest of the time.
Where should you put them?
Either severely reduce the chances of finding a cupcake room and keep that in the game, and change the prize to 1 temporary cup cake helmet/confetti
OR
Have them replace all monsters, at a rate so low that they'll be harder to find than love puppies. Given that love puppies cannot replace more than a select few monsters, whilst these would replace all mobile mobs, said rate should be phenomenally low. But still there, as an easter egg. Erm, cupcake.
If anything, I'd think they'd replace Jelly Cubes.
I'm not too keen on the idea of having them drop temporary cupcake helmets, though. For whatever reason it just doesn't seem right to me that you'd start seeing people randomly walking around with them.