Here is the full list of all 71 Mask Up achievements.
Unlock the Unknown mask
Unlock the Bear mask
Unlock the crying mask
Pet 3 dogs in a run
Unlock the dirt mask
Unlock the peace mask
Unlock the sleep mask
Find the Pipe mask
Find the lab mask
Find the plant mask
Find the stoplight mask
Get to room 200
Unlock the Vener mask
Complete a "true pacifist" run: Pet a dog without killing anyone.
Unlock the arrow mask
Unlock the body mask
Unlock the sugar mask
Unlock the scarlet mask
Unlock the dino mask
Unlock the hat mask
Unlock the book mask
Get lucky
Reach the second biome in neutral mode
Get the cat back to its owner
Unlock the fish mask
Spare Normal Guys until the statue room
Spare Shoot Guys until the statue room.
Spare dogs until the statue room.
Spare Heart Guys until the statue room.
Spare drones until the statue room.
Bring a mecha-dog to the statue room
Buy the reverse mask
Buy the wood mask
Buy the neon mask
Buy the stone mask
Unlock the painting mask
Kill a mecha-dog with the shotgun
Kill an enemy with the gong
Bring the cat to the piñata
Find the dodo mask in the credits
Spare juiceboxes until the statue room
Don't get hit until the statue room
Do a high score in flank mode
Hit 3 enemies in a single katana hit
Kill a mecha-dog with the katana
Grab the alien mask
Go in the purgatory.
Reach room 200 with wings
Hit a bomb with the gong
Corrupt the statue room
Beat The Fox.
Stay in first form until the statue room
Kill an enemy with a statue
Corrupt a mask
Uncorrupt a corrupted mask
Feed a drone enough juice, then kill it
Get three mask together
Buy a mask from a corrupted dispenser
Unlock the nose mask
Box a mecha-dog
Unlock the bottle mask
Get extremely corrupted
Get two mask together
Unlock the happy mask
Unlock the onion mask
Unlock the flower mask
Unlock the staff mask
Unlock the caregiver mask
Unlock the fire mask
Crack the egg
Beat the fox in the true fight