Yes, morale bonuses stack. However, the calculation is a little bit more complicated.
Each planet has a base approval, ranging from 10% to 100%. The base approval determines how effective the bonuses from morale improvements and your morale ability are. How high the base approval is, is determined by the population.
In your case, the population reduces the base approval by -75%. This means, that the bonuses from your morale improvements and ability are only 25% as effective as the could be. You probably have build one of the extreme stadiums on a bonus-tile, or have another building, that increases morale. Otherwise, the 20% from buildings makes no sense (3 x 20% = 60% x 25%= 15% not 20%).
Your population is also above 20 billion, which is a bad idea, because it takes a tremendous amount of morale improvements to keep that many people happy. It just isn't cost-effective, because you pay more in maintenance than you get back in taxes. Just get rid of some farms, so your population is either at or slightly below 20b.
Two more things to note. No matter how high your morale ability is, it can never provide a bigger bonus than +100%. The high quality planet bonus is not affected by the base approval. It is always +10%.