Going "Mashed Potatoes" on someone is where you perform the act of pulling down your partner's pants, smearing mashed potatoes (any flavor will do) onto their buttocks, and "motorboating" them between the cheeks. If done correctly there will be mashed potatoes strewn about the area where the act had taken place.
Tom: "Check out that girl in the yoga pants", Earl: "Oh yeah, if I had the chance I'd be going mashed potatoes on her all day"
Charlie Potatoes is an old British and American slang term for a man who is on top of the world in terms of either money or popularity. In the film "The Defiant Ones," the escaped convict John "Joker" Jackson (Tony Curtis) fantasizes about a time when he will be rich and popular, calling himself "Charlie Potatoes" when he talks about it.
"I'm gonna buy me a pair of buckskin shoes, with a brand-new suit and a silk shirt. And I'll be CharliePotatoes, comin' down the street, with a Panama hat and a good-lookin' gal."