Is this a good reason for a 'goddess' to become 'mortal'?

This is my take on it, and you don't have to listen to me, as it is your story.

You mentioned she chooses to become mortal because "She's bored". That's actually not that farfetched of an idea if you can sell it to your readers.
Given that she is a Goddess, and one of the oldest, one can assume she's seen quite a bit of life. That as the embodiment of the Earth element, she's paid attention to humanity and the world, and how it changes. Unlike her own realm. Being immortal, nothing really changes, and it seems like the other Gods/Goddesses don't have any intentions of changing either. They've all accepted their immortality and just go with the flow. Except her, she wants something different in life. She wants to experience the uncertainties that humans experience every day. To her, their life is exciting and adventurous, and she's envious of this. She wants to experience that firsthand instead of being "bored".
So how can she achieve this?
You mention the other Gods/Goddesses are a crafty bunch, and she is rather naïve in her own right. Perhaps one of the other Celestial beings convince her that she can experience the human life she wants by casting x-spell. Selling it by telling her she can retain her godly powers even in mortal form. However, they don't tell her about the drawbacks of the spell. The biggest one being, that if she were to die in her mortal form, then she can never return to being a Goddess.

Just something to think about.