Going by the analysis in the post, and in the parent comment, the 20mm would probably be positively correlated with attractiveness because it's easier to get a shallower depth of field with it.
However, you're certainly correct that conventional photographic wisdom is that people look better on longer lenses, for just the reason you cite -- wider angles can distort faces unpleasantly. It would have been interesting to see an analysis of lens length & perceived attractiveness from this dataset.
You have to be too close to the face in order to throw out the background with a 20mm 1.9. It's going to make the nose look weird and accentuate a double chin, if you have one. (I have the same camera and lens)
However, you're certainly correct that conventional photographic wisdom is that people look better on longer lenses, for just the reason you cite -- wider angles can distort faces unpleasantly. It would have been interesting to see an analysis of lens length & perceived attractiveness from this dataset.