Less attractive faces carry more information (visually) because they are not as symmetric, have distinguishing blemishes, malformed shapes, or whatever, and the

Less attractive faces carry more information (visually) because they are not as symmetric, have distinguishing blemishes, malformed shapes, or whatever, and therefore trigger more neural pathways than the "average" face does, and therefore makes them easier to recall. Each abnormal aspect of a face triggers a neural pathway to remember that attribute, and the worst faces therefore trigger the most pathways. And each pathway improves the statistical probability of a future recognition (recall).