My Little Pony: Friendship is Magic - so, scootaloo has enough wing power to pull herself on a scooter and 3 other ponies in a cart but she doesn't have the wing power to fly on her own?

Welcome to physics!

Rolling a wheeled object along the ground requires far less force than lifting that same object into the air, not to mention that the aerodynamics of the wing motions would be pretty different, too.
(Also, Pinkie jumped in while they were already moving, so she would have been relatively easy to pull as the momentum was already there.)

The cart has wheels, specifically to reduce the friction between the ground and the cart.