According to Forrest Wickman, it wasn’t until “the late fifties and sixties, motherfucker finally became, in some usages, a positive description”:

People have been calling each other motherfuckers for over a century, but until World War II the term was typically used as an insult. The earliest citations of motherfucker and motherfucking in the Oxford English Dictionary come from the late 19th and early 20th centuries, and they give a sense for how seriously the word was taken. The OED’s first citation of the word comes from the Texas Court of Appeals’ account of the 1889 trial of Levy v. State, where witnesses describe a defendant being called a “God damned mother-f—cking, bastardly son-of-a-bitch.” It’s perhaps revealing that, of the four expletives, mother-fucking is the only one to get censored. The Texas Court of Criminal Appeals apparently felt more comfortable printing the word just a few years later. Their records from 1898 include an account in which the word is offered almost as grounds for murder: A defendant suggests that he should be partially excused of killing a man just because the man had called him a “mother-fucking son-of-a-bitch.”