I recently removed the following paragraph from the text describing Sweetie Belle's personality:
Spacing out at times
On several occasions, Sweetie Belle seems a bit slow on the uptake, pausing for a moment before catching on. When the CMC are at Fluttershy's house in the episode Stare Master, Scootaloo and Apple Bloom pretend to be a monster from the Everfree Forest. Sweetie Belle meets their playfulness with a blank stare for a moment before joining in. Several times in The Show Stoppers, she asks her friends to explain to her what's going on and announces the nonsensical "no meal uncooked" as one of the CMC's goals. While accompanying her sister, she takes the phrase "bitten off more than I can chew" literally, responding with a confused "but you're not eating anything." In Lesson Zero, when Twilight Sparkle tries to force the Cutie Mark Crusaders to take her Smarty Pants doll, Sweetie Belle can only come up with "I really like her mane" three separate times as a response. Still, Sweetie Belle manages to recognize Apple Bloom's grammatical error in The Return of Harmony Part 1, pointing out to her that "Victoryful" is not a word, mirroring their sisters' discussing about the word "Get-alonging-est" in Look Before You Sleep. "
First three examples = just childish naivete on her part. Come on, guys.
Doll example = she ran out of things to say.
Victoryful example = ok, let's say she's more well-read, but there is no valid conclusion we can draw.
I really think there's nothing to suggest that anypony is particularly smarter than the other, anywhere in Seasons 1 and 2...