In DA (i assume the same is true for ToA) 13B is the optimum point of population V income V morale. You use 2 tiles to achieve the best balance, using one VR centre and one ADV farm. But morale is also directly affected by your resources as well. If you can manage to have 4~6 morale resources then a 20B planet becomes very viable on PQ15 worlds or higher. You can go to 20B on as little as a PQ9 if you wanted to but the trade off of tile usage to maintain the morale Vs potential income makes this unfeasable.
PQ15 and higher would need two ADV Farms and two VR centres to maintain your 20B and you could push your taxes to around 69% or 79% very easily. This all depends on what other racial bonuses you may have or the like.
Your morale will affect your score in that it is directly linked to your income and population. Obviously early game you want to maintain 100% morale for the 100% bonus to pop growth. (even more so if you play as Torians) I always research morale improvments before farms to ensure i can always maintain that 100% for the first 12 months. Later in the game once your planets have reached maximum capacity then push your morale down with higher taxes, morale matters only for voting by this point in the game and for countering culture flipping, both easily accounted for.
Keeping morale above 41% will ensure that your planet will grow, given enough food but without bonuses. Above 75% you get 25% bonus...IIRC.
Always try to minimise the amount of VR centers you have to build. Two per PQ15+ planets along with the two ADV farms and the rest of the tiles econ will give you the best balance you could hope for between income Vs morale, of course getting those morale resources is the key.
In gigantic abundant all games i have had enough morale resources that i was able to run at 79% with only 1 VR centre on a 20B world and keep very high approval.
Like Noct said it all depends on what style of game your playing and what map. On smaller maps you are going to be limited to a smaller number of galactic resources so the 20B planets may have to be reserved for your PQ 18+ worlds.
Leave your 20B worlds to later in the game...keep your morale high for the growing phase then expand and let morale drop to ~45%.
Green approval on a planet means your probably not pushing that planet to it's income and population potential.