My mom's exasperated "shit a fiddle!" when fed up with something / something broke. When I was younger, she didn't really say curse words around me except for this.

I've never heard any one else ever say this. Not in Appalachia, or anywhere. She probably made it up herself. But in the 80s she also dated a Korean War fighter pilot/POW (crashed, survived, & captured, unsure of release details). And he could have had a creative catalog of swears that she borrowed from.