Sketch of Pinkie Pie for 2008 MLP:FiM pitch bible. With Pinkie working at the Sugarcube Corner, I imagined she had waaaaaay too much access to sweets which would send her wings into hummingbird mode. You\'d probably want to stay out her way in this state.
I think it would have fit her wild, eccentric nature perfectly, but as an earth pony it's even funnier when she somehow reaches impossible places, so yeah, I see where you're coming from. Fluttershy would originally have been an earth pony, which also would have fit her nature better. But as a pegasus, constantly forced to fly against her will she's much more interesting.
In case you were wondering, the stated reason was that Pinkie and Rainbow would have the same flight patterns- in the air all the time, being way too fast and way too crazy. It would also imply too much about species stereotypes (all pegasi are pranksters and all earth ponies are super good with farm animals), so they gave the least fast/crazy/prankster pony the job instead.