Oh shit...I had mashed potatoes!

This is unfair to your mother because the laws of angles prevent her from rubbing her genitalia in the mashed potatoes. Instead, she must go through the laborious process of scooping out mashed taters into her vag(ina) and then depositing them back in the bowl, introducing the possibility of mashed-taters-transmitted…

No, you should never tell her. Every time she nags you about something, think of this, and laugh. It will drive her nuts.