Although I don't see much use for the word in comedy, "m-----f-----" is way more transgressive and ugly than "fuck", because of the connection with rape and incest. To call an enemy a "m~" is to accuse him of incest, hitting on one of humanity's oldest taboos. To describe oneself as a "badass m~" is, often, to threaten seduction or rape against others' mothers-- in other words, to dress oneself as a dangerous and indiscriminate sociopath.

It's probably one of the most intrinsically ugly words in the language. Racial slurs are ugly because of what they represent-- hideous and unacceptable attitudes held against innocent people, based in prejudice and ignorance-- but not on account of composition, because they're usually atomic words. These words become extremely offensive once one knows their meanings, but beforehand hold no special currency. By contrast, "M~" is a compound of two well-known and fairly innocuous words-- one completely acceptable, one edgy but mostly inoffensive-- that instantly makes the skin of even most liberal people crawl, even before its semantic subtleties are explored.