Here is the full list of all 16 Monsters Ate My Birthday Cake achievements.
Find All Message Bottles
Find all the monster companions
Free Lil' Willis 10 times
Complete Apricot Acres
Open 25 treasure chests
Complete Nogport Meadows
End the Boogin King's reign
Complete The Deadlands
Complete all of Poe's requests
Collect 150 Stars
Collect 5 costumes
Get 5 'Spare all Boogins' challenges
Use Poot to toot on a Boogin
Complete Powder Peaks
Freeze a Boogin into a block
Eat 100 pieces of cake