Okay, I've done some testing on the tanning and I'm convinced that Sims need to be in swimwear to get tanned or burned.
Despite sitting outside in the sun for three full days in everyday clothing, my Sims did not get tanned or burned. One of the Sims got the Sweating Profusely moodlet, but no sunburn when in everyday clothes. The other had the Loves the Heat trait and had the Starting to Sweat moodlet, but no tan or burn. The next day, they were again sitting outside, one in the mermaid costume as swimwear and the other in everyday clothes. When the one in the mermaid costume got the Sweating Profusely moodlet, I had her change into a more traditional swimsuit and sit back down. She hopped right back up with a nasty sunburn. I then had the other Sim change into his swimsuit and sit back down, and he hopped up out of the lounger to show off his nice new tan. So it seems that everyday clothes keep a Sim from getting a tan or a burn and that the mermaid costume, even when assigned a swimwear, acts the same way.
