As devil fruits are the deepest desires of humankind, and also have a drawback, i.e., one cannot swim after eating them, it makes sense if devil (Imu) is the creator of the concept of devil fruits. Providing powers for a price is a core concept of the devil, domi reversie is also another example. Also, as the world sinks and there will be an ocean after the 3rd Great War, devil fruit users will be at a great disadvantage, so it is actually a much bigger drawback than just not being able to swim. Also, the mural states that the devil fruits emerged from people's greed for more power. Now the question is: if the zoan devil fruits like the Nika fruit have a will of their own, why would Imu create those? I think Imu just invented the concept of turning desires into devil fruits, and Sun God Nika emerged from people's hopes, as stated in the mural. 

On deeper thought, I think it might be possible that it is all a reference to Eve eating the forbidden fruit, and that fruit is the og devil fruit that started all this: "Mankind succumbed to greed and touched the forbidden sun". It would also reference the hellfire serpent as the one that influenced Eve to eat the apple.

Here is the final thought: initially, there was the earth god (biblical god), and the serpent. Then, after eating the forbidden fruit, the desires and greed of people gave rise to the devil, who tamed beasts: "Forest god tamed demons". Or Imu might also be the "man killed the sun and became god". Then in the 3rd world, Nika fruit arises again. Sea god is Davy Jones 

Imu is the creator of the concept of devil fruits.