In shoujo anime and manga such as Vampire Knight, Special A, and Kaichou wa Maid-sama, the male love interests (or Zero in the case of Vampire Knight) are shown to be top scorers in the class. Is there a known reason for this to be so common?
In stories—regardless of the medium—the main characters invariably have at least one stellar or likeable quality. They might be really smart (id est, top scorers), highly talented, very popular, really funny, or simply very hard-working. If a protagonist does not have such a quality, s/he will develop it as the story goes along. This is simply the way feel-good story structures work and shoujo and shounen are all about feeling good.
Shoujo's target audience is girls and therefore the male protagonists are often one or more of rich, good-looking, successful, talented, funny, etc., etc.; all qualities that girls typically like in boys. They are not always top-scorers: for e.g., in Kimi ni Todoke, it is Sadako who is specifically shown to be really smart while Kazehaya is more of the popular nice guy, or IOW, he is based off a different archetype. You won't normally find the female lead falling for a loser male in shoujo; that is more likely to happen only in josei.
Asian cultures do tend to place more importance on academics than Western cultures. I expect that the fact that there is also no real equivalent to anime and manga which are set in schools is also a factor on why traits such as academic excellence don't register as much. That said, novels such as Lucy Maud Montgomery's Anne of Green Gables do use academic performance as a character attribute or plot point. It is no surprise that Montgomery's book has been adapted into an anime, Akage no Anne.