To have a diverse and interesting population of sims, I don't try to create any from scratch or download any from the Gallery.
Instead, whenever the game generates a new sim, I assume there must have been a reason it was made (for example it might have been made to fill a job role, or to fill a deficit in sims with certain traits, or to fill the need for sims living in a region e.g. San Myshuno district townies, etc). If I made my own sims and placed them in the game, then the game would probably ignore them if they don't fill whatever reason the game needed.
So instead I take that new townie the game generated into CAS and use them as a base to edit them to my own desire. Most of the time, I can do whatever I want, such as change the race, hair colour, clothing, name. So, if the game made yet another blonde Japanese sim called Genji Sakamoto, I can totally give them an extreme makeover and turn them into a brunette Russian guy named Dimitry or something more diverse.
For example, when the game generated 18 new sims to work at Plumbob Pictures studio when I made one of my sims an actor for the first time, they were mostly all the same generic townie. But I went through each of them and gave them some more racial diversity, backstory, styling and pizzazz and ended up with my own
custom studio crew.
Sometimes, when the game made two different townies with the same surname, if they looked similar enough, I merged them together and made them into a family group. E.g. I put two elders with the same surname together and made them brother and sister. Then in game when I was playing my own sim at the Police Station, I saw those two elders and thought it was pretty cool that they worked together - the elder guy was actually the chief of police and his sister was one of his police officers. Then she walked into his office and gave him a sisterly hug. It's those kind of moments which made my sim world seem more connected and interesting.