"Your mom", sometimes also "yo momma", generally depicts an obese, unemployed, permissive, poor, unintelligent and ugly person. The phrase is usually NOT literally meant as a direct insult to a person's mother, but is supposed to describe a type of imaginative fantasy person with the said characteristics. When visually depicted, "your mom" is usually ugly, neglected or absurdly obese. These attributes are seen in the intro of a maternal insult, e.g. "Your mom is so fat,...", but sometimes occur later within the joke or even deliver the punchline itself.
--Please control your sensitivity, and know what JOKE means.
Also, don't cover your provocation.