The Sims Mobile game introduces the lifestyle mechanics. It unlocks additional rooms and living spaces in your area. The quality of life is defined by the symbol at the top of the screen. Depending on the level, it differs in size and color.
The first method to increase the quality of life is to buy furniture and also to further expand your house. If you expand your area and improve it, you will gain additional points. Usually the number of points depends on the price of the object - if the object costs 800 you will get 800 quality of life points.
Another way is to buy clothes or get them at Izzy's Boutique. All you need to do is change your sim clothes frequently and quality of life will increase quite fast.
The last way to improve lifestyle in The Sims Mobile is to get some heirlooms. Each souvenir is worth about 1000 quality of life points. You can earn heirlooms by sending your Sims to retire. You can also buy them at the store. The description of the Heirloom Store can be found on a separate page.