I'm surprised people hadn't heard the word "goomba" before; it's one of those words that people think Italian-Americans say all the time, but really they don't say it that often -- but still often enough that the average person knows it. Being born in Brooklyn, Mario's home, I heard it all the time as a child, and being Italian-American, Mario would naturally use this word.
Do you use the word "compadre" in Spanish? "Goomba" comes from the same word, but in Italian. Many English speakers cannot distinguish unaspirated consonants from voiced consonants (that is, they hear unaspirated "c" as a "g"), so you get words like compa as "goomba" and, lopping the vowel off at the end, stu cazzo as "stugots'" and (va) fan culo as "(va) fan gul'"
(Sorry for the vulgarisms, but I can't think of any "innocent" examples!)