Sore Thumb Factor (or STF, sometimes also called Stand Out Factor) occurs when one entrant in a 4-way match (or other large multiway poll) "stands out like a sore thumb" against their opponents.
This factor was brought to light most prominently during the Summer 2008 Contest, when Kirby was one of many characters who had STF work to his advantage. He repeatedly showed up in hilarious contrast to other more "mature" contestants, leading to his eventual advancement into the semifinals.
We'd also seen STF play a major role the previous year, when Pikachu went on a similar upset run by overperforming against "mature" opponents, such as his first place finish against the Capcom gang (Dante/Leon/Ammy).