Was it common to eat mashed potatoes out of cones in the '50s?
I would guess that this is just something that Schulz thought was funny, and wasn't part of a larger trend. Saw some prank videos on youtube where people were handing out mashed potatoes in cones instead of ice cream at parks, so probably just a common prank idea.
In ice cream commercials, they sometimes use shortening (e.g. Crisco), since actual ice cream would quickly melt under hot studio lights.
For posts about the comic Peanuts, by Charles M. Schulz
