I know this game is way past it's date, however I felt I needed to add a guide since all the guides here miss out the most important thing - to define what is needed for the achievement!
Ultimate Humiliation achievement - Perform every fighter's hidden finishing move
This requires you to perform Each character's Babality on any opponent. This is not viewing each character as a baby, rather making an opponent a baby.
NB: To perform a babality, you have to complete the match without blocking! As stated in other solutions, this is best done on Beginner mode, setting the match timer to 30 seconds and one round.
Most of the babalities need to be within jumping distance (this is just far enough for a sweep to miss, or a back flip away from the opponent)
Baraka: Right Left Right
Cyber: Down Left Right
Cyrax: Down Right Left
Ermac: Down Down Left Down
Jade: Down Down Right Down
Jax: Down Down Down
Johnny Cage: Right Left Right
Kabal: Right Down Left
Kano: Right Down Down
Kitana: Right Right Down Right
Kung: Down Right Down
Liu: Down Down Down
Mileena: Down Down Right Left
Ni: Right Left Right Left
Noob: Right Up Right
Quan: Right Down Left
Raiden: Down Left Down
Reptile: Left Right Left Down
Scorpion: Down Left Right Down
Sektor: Left Down Down Down
Shang: Down Left Down
Sheeva: Down Down Down Left
Sindel: Down Down Down Up
Smoke: Down Left Down Right Down
Sonya: Down Down Right
Stryker: Down Right Down Left
Sub: Down Left Down