Don't know, the Magikarp one is pretty bad as well... At least the rats give you some decent XP and are more common where I live.
Caught 508 Magikarps, only 68 of which were XL (~13%).
Caught 1354 Rattatas, 144 of which were XS (~10%).
Without the holiday Pikachus, the Pikachu medal was pretty far out for me. It is closer now, but such a grind...