True, it isn’t uncommon in southern Indiana, particularly in the outlying areas. Like Daviess County, where my former in-laws live. Nice people, but a wee bit backward. 
I’ve also heard it in some of the more desolate parts of West Virginia.
My mother calls her mother ‘Ma’, don’t rember what she called grandpa. Northern Wisconsin.
I actually call my mother ‘ma’ sometimes (and not as a joke) – I picked it up from an Italian friend in grade school in Buffalo.
No I don’t use it to be funny. I use it naturally. But I cannot say that I have been raised to do it.
As for my dad’s family either Mommy, even as adults or Mother. Not Mom or Momma.
Just remembered my brother-in-law does call his mother Ma. He does not do it to be funny it is what he was raised to call her. He calls his father dad not Pa. He is from Cleveland, OH.
I was partly raised in Pennslyvania and my sister and I always called out mother “Mum” and addressed dad as “Daddy”. Also we addressed our aunts as “Ants” as in “Ant Mary”. Grandmother was “Grammaw” and grandfather was “Pup pup”.
(Well, we were kids!) 
In Mississippi, your mother was mama, and your father was daddy. Sometimes grandmother would be meemaw. I only ever saw “Ma” and “Pa” in the Little House on the Prairie books.
In Turkey, there’s a list of grandparent’s names, including baba-anne (father’s mother) and anne-anne (mother’s mother). Interestingly, the word for cousin, kuzen, is a recent import. I asked an older Turkish lady and she said that before everyone just said “aunt’s son” or “uncle’s daughter”.
Interesting input everyone, but I’m having a hard time discerning any geographic patterns here (or at least any uniformity). I see a preponderance of “Momma (or Mama) and Daddy” among Southerners, but even within the South (and even within specific states) there seems to be considerable variability from family to family.
I wish some native West Virginians and eastern Kentuckians would check in.
My wife’s maternal side of the family, based for many generations in the hill country of southeastern Kentucky, use Maw and Paw to refer to grandparents. Younger generations (current middle-agers and younger) use Mommy and Daddy for parents, but their parents’ parents are always Maw and Paw. I gather that this was a generational thing. The older generation, bearing children in the twenties, say, were referred to by their children as Maw and Paw. When the next generation came along (born in say the forties and fifties), those “names” were already taken for those specific people (i.e. their grandparents) and they used Mommy and Daddy for their parents. In any case, to my wife (and her kin) “Maw” and “Paw” refer to specific people (my wife’s maternal grandmother and grandfather). But I suspect this is dying out (Maw and Paw have been dead for years), and eventually may simply be a bit of family history and no longer used. To our children, my wife’s parents are Mamaw and Papaw; to my wife they’re Mommy and Daddy; but to everybody, her grandparents remain Maw and Paw.
Side note: the spelling of “Maw” and “Paw” does not convey the pronunciation very well, really. Being a born-and-bred Yankee, I have never been able to duplicate the dialect very well (nobody in those parts would ever be fooled into thinking I was a native), and even my wife is losing the ability to speak genuine Kentucky hillbilly, living among Yankees all her life and speaking “south-ren” only rarely over the past thirty years. But “Maw”, as spoken in Whitley county, Kentucky, has a drawled, slurred diphthong, in which a flavor of a long “O” sound slips in somehow (to my ear).
It’s a fascinating accent (dialect), but in the decades that I’ve been associating with people in that region, the accent seems to be weakening, softening, and inching away from it’s deep distinctiveness with each new generation. I think broadcast media are eroding it. In my (admittedly amateur) opinion, it isn’t likely to disappear completely, at least anytime soon, because it’s very much a cultural marker and a point of pride for many (most?) of the people there; I believe they cling to it as a part of their identity, and I’ve heard them “step it up” and deepen the drawl unconsciously (or consciously) when talking with outsiders. I believe they take pride in it. But they’re also capable of laughing about it among trusted outsiders, as well as laughing at silly Yankees (and actors on television or film) who try to imitate it.
I’ve seen “Pa” for Dad in at least one working-class British English dialect, at least judging by a particular P.G. Wodehouse short story.