The mother often represents morality and good intentions—which is both good and bad for portrayals of women in music.
Miranda Lambert's most recent single, "Mama's Broken Heart," lurches between a sparse, echoey, hiccupping lope and a rock-punk wailing shriek. The bipolar dynamics mirror the central conversation of the song, which is not between lover and lover, but between daughter and mother, the second advising restraint following hearbreak ("fix your makeup, girl, it's just a breakup, girl/hide your crazy and start acting like a lady") and the first enthusiastically embracing her self-parodic hyperbolic Shakespearean despair ("I cut my bangs with some rusty kitchen scissors/I screamed his name till the neighbors called the cops.")
Lambert's mama, in other words, performs the same function as many a mama in country music—she's the voice of reason and social morality ("Word got around to the barflies and the Baptists/My mama's phone started ringin' off the hook") against which the singer gets to rebel. Examples abound. There's Johnny Cash's brutal, stripped-down 1958 single "Don't Take Your Guns To Town," in which Billy Joe ignores his mom's warning and ends up dead on a barroom floor. There's Merle Haggard's 1968, "Mama Tried," where the shimmering Bakersfield guitar rolls on irresistibly out of mama's arms, onto the rails, and into that prison cell. "That leaves only me to blame, cause mama tried." And of course there's Waylon and Willie's self-mythologizing "Mamas Don't Let Your Babies Grow Up to Be Cowboys," which straddles the line between joking sincerity and sincere joking as it describes the mama-alienating aspects of the cowboy life ("he ain't wrong he's just different/but his pride won't let him/do things to make you think he's right") Those mama-alienating aspects, of course, being the same things which make the cowboy life attractive.
Country's obsession with mamas is part and parcel of its obsession with tradition. It's one of the many ways that the music evokes and points to the rural past which it both constantly idolizes and constantly (though sometimes surreptitiously) celebrates its liberation from. Mama, then, becomes a convenient trope; a way to declare one's love for that old-time morality while emphasizing one's own tragic/sexy/cool/funny independence from it.