It's a misspelling of the Italian interjection "mamma mia!", which literally means "oh my mother!".
It is used by italians in quite tha same way as English use the expression "oh my God!", except ...Italians do NOT say "mamma mia!" when they're making love with their girlfriend!
"Hai visto quel film horror?" "Mamma mia!! Era così spaventoso!" (= "Have you seen that horror movie?" "Oh my God!! It was so frightening!")
There's a musical called "mamamia": is it about Italian mafia?
An exclamation of surprise, often uttered by those of Italian ethnicity.
Mamamia! That's-a one good pizza!