The shape is just a result of the engineering decisions to reach Airbus's goal: transport fuselages to Toulouse. Those are relatively light, but big, so building a tube-like structure above the main "chassis" of the plane yields to this kind of shape, which we then recognize as a similarity to beluga.

Ah, so it was all after-the-fact. Thank you.

It certainly is not functional. It cannot fly in air. Like penguins, it can only 'fly' using a liquid medium. Hence the nickname. Coming soon to a river near you.