Jo Mama is a cross of psuedo-ebonics and a semi-fictional character. It is a slang colloquialism.
Its roots lie in the "Your Mom" jokes which have gained striking popularity over time. It is frequently used in a knock-knock joke:
1: Knock, Knock.
2: Who's there?
2: Joe who?
1: Jo Mama.
"Jo Mama" is generally considered to be playfully insulting. By speaking the phrase in a contrived accent, one can easily indicate that one is joking, as opposed to saying "Your Mom" in a flat or aggressive tone.
See also: Your Mom