The Planet Quality morale bonus applies for all planets that are, or have become, a class 11 or higher. As the homeworld starts out as a class 10, you can give it a morale boost by researching Soil Enhancement and converting the yellow tile into a green one to make it a class 11. However, anything above that doesn't matter for morale. Also, even the +10 morale isn't enough to make a population of 20B or more financially attractive. Do not cross the threshold of 16B for good planets (except perhaps if you have several approval bonus tiles, but even then it's iffy), it simply doesn't pay off if you want to make as much money off your population as possible.
