It's a Naruto reference (Naruto the T.V anime show).
In the first arc, Naruto had a battle with a boy named Haku.
But Haku looked like a girl - since he had long black hair, big brown eyes and wore a cute pink kimono.
And so when Naruto found out, he said that Haku was, "prettier than Sakura (a girl Naruto had a crush on)!!"