Where I am from in the South (Alabama), yes it is a thing. We did eat them from time to time growing up as kids (1980s I wouldn't go so far as to call it a staple more like a mom needs to go grocery shopping, here kids we have mayo and some bananas and bread type of thing). The main key is to use just a little mayo (about 1 1/2 Tablespoons tops) and a very firm banana. If you use too much mayo and/or a banana that isn't firm enough it's just nasty mush. I haven't had one in years (because if I had rather use those calories for something more filling plus I have banned mayo from the house) but if you make them the right way they don't taste bad. Of course she is probably using half a cup or more of mayo and mushy bananas.