Reading the twitter comments, this image was apparently inspired by a discussion about how (male) artists frequently draw girls pulling their skirts down to use the toilet as if they were wearing pants, instead of just rolling them up. The example was given of a girl wearing a one-piece dress, would she take the whole thing off? Obviously not, as this image shows how silly it would be.
There are a few reddit threads about that question (from a google search). Majority of the responses were definitely in the up direction with the minority in the down direction. Seemed to really come down to it depended on which direction was easier or cleaner to the wearer. There were at least a few who stated they change depending on how clean the bathroom is (clean home/friend's bathroom down, dirty public bathroom up) or for example how long or tight their skirt was (short up, long or tight down).