Athena, mummies are not appearing related to VISA level. You can find them in China in two tombs (Terracotta army tomb and Dragon Cave) but you need adventure to reach them. In Egypt there is much more of them, and they are all in sarcophagus. They appear after you inspect sarcophagus (I think they can appear if you inspect some chests or enter some special areas too).
Now, there is nothing to be worry about. It is recommended that your Sim has maxed both martial arts and athletic but it is not necessary. I had a situation where mummy cursed a Sim who was there only to take a picture of explorer who fight with it. When curse happen, you have many many options. You should travel to Egypt if you're not already there, and look for adventure Soaked, Burned, Cursed (or something like that) and that will lead you to the Sphinx where you can plead to soulpeace statue to take that curse off. Also, you can practice that snake charming. If nothing work, you have several cheat options (you do get 14 Sim days so there's plenty of time). You can put testingcheatsenable on and Click of the moodlet, or pace soulpeace statue find in buydebug and plead to it in your home, of simply, harvest deadflower from graveyard and wait for Grimm to give it to him.
I fight with many many mummies, and only 2 times got cursed situation. Once with Sim I mention, who was photographing tomb exploration, and once in Egypt even if martial arts and athletic was maxed.
You can beat a mummy and get cursed, you can run away and still get cursed. But still, it is rare and if happen, it's nothing to worry about.