Obviously Rule of Funny was in effect for the actual scene (as well as Overly Pre-Prepared Gag), since the whole prospect of Rainbow Dash standing for two days straight inside a snowman, while funny, is unrealistic: In real life she would have gotten frostbite, she would be fatigued from not eating or sleeping, someone like her SHOULD have been bored out of her skull standing still doing nothing for so long, and of course, without any trips to the bathroom she would have wet herself at least twice.
 
While she probably unrealistically stayed dry as of canon as fictional characters often do in similar situations (like how the ponies somehow never had to pee once while locked up by Starlight due to lack of desperation or puddles), it is fun for me to imagine she did wet herself there (that she was THAT determined for a sneak attack, plus she could have drank energy drinks to stay awake while just enduring the cold and hunger, which of course would make her need to go more), especially since later in the episode she uses going to the bathroom as an excuse to find a present for Fluttershy last minute.
 
Alternately one could imagine her holding it for two days then wetting her pants when the snowball hit her. (I imagine she’s wearing her skintight black pants underneath her snow pants)
 
Who knows, they did show Fleur at the back of a bathroom line looking desperate not too long ago…