Pixi, on a general level, my answer is no. Some even say it's harder to stretch long body parts as it's more material to stretch out. Maybe so, but I think that factor becomes small in comparison to
everything else that regards being successful in stretching (like the condition of our joints for example).
I will say that long legs can APPEAR both less and more flexy depending on the situation. If you are flexible enough to oversplit, long legs will be advantagous. For example imagine two people with different leg length doing oversplits on the floor with the front leg elevated. For the same angle between the floor and the front leg, the leggiest person's foot with automatically be at a higher distance from the ground than the other person's foot. That I'm sure will create an illusion of more flexibility for the leggy.
But this also means that until you have full split, the long-legged person will instead have a disadvantage, for the same reason. Imagine a Jade where the legs aren't perfectly splitted. The foot of the leggy one will be at a further distance from the horizontal line and probably appear less flexy. It's all geometry here.
The weight and lever arms of the legs can play a bit of a role sometimes, according to laws of nature. Take that Jade again. Gravity will work harder on the leggy persons legs and help them a little more to split. But in Chopstix it's the opposite again who gets the advantage!
Then like Veena said, you can learn how to appear by using smart angles etc as well as straighten the legs and point feet to the maximum. And I think that since this is an ability that grows with practice, and flexibility also grows with practice, these often go hand in hand. That could partially explain why you correlate flexy to leggy!