Hi, Mesh Modeler dev here.
i'm sorry to say that, so far, there is no possibility to reduce automatically the number of faces on your sphere.
In a future update, items will be 30% smaller so i hope it can help solve your issue.
Also, you may have a workaround until then : make your sphere very small, the MM will make it have not many faces. After that, you can select all your sphere and resize it to make it the size you want with still having as many faces as it had when small. I know it's not a perfect way but it may help ^^
Also, like eFFecT said, if you want to use it in a map, you have to go through the item mode to make an item directly out of your mesh. Or else, you can export it as a crystal like you've seen and in the item editor import it as the "Visual Mesh" field ^^
While you have it as a Crystal, you can actually use your Crystal to import it in future meshes easily (as a prefab) to help you build more easily.
I hope my words helped you and thank you so much for using this tool, it means a lot
Niebo