Rainbow Dash was inspired by a, at the time, 4 year old girl named Harper. As you might imagine, she loves rainbows and My Little Pony.
The attempt to create a game that a 4 year old could enjoy while still also being enjoyable for adults was another way she influenced the design. It is for this reason the levels are designed to be easy to beat but difficult to 100%. In this way there are multiple layers of challenge to provide gameplay for all age ranges.