The mask demasks Ipkiss' mask
When Dr. Arthur Neuman is speaking on the TV as Ipkiss is watching, he talks about his theory of social masks and that everyone builds one to conform to established societal rules.
The Ipkiss we see before he gets the mask is exactly what Neuman describes: a people-pleasing push over who aims to appease anyone and everyone before appeasing himself. Sacrificing everything of value from himself and placing it all for other people.
The Mask destabilizes his existence by taking the direct opposite of Ipkiss' mask. Radical, rule-breaking, risk-taking and all unflinching with decision and desire. The Mask is fearless.
After each scenario with the mask on, Ipkiss develops a strong character that helps to facilitate, express and control his own personality in the real world. Therefor, establishing a new-found self-esteem to become the person he wishes everyone could see without any fear.
This is what I got from the movie.