Mom is a disaster. Unless she pretends to be a rip-snorting paleo, anything that comes out of her mouth is likely to be a disaster.
They're bringing her out in the hopes that Trump will trash-mouth her. That wouldn't go over well in the South, and the Bush family knows it. They're desperate, and she's Bait.
It wouldn't make people switch their vote to Bush, but it would be enough for them to switch to Rubio.