Is it possible for someone without any training in martial arts to defeat an experienced fighter using any type of martial arts?

Yes actually. It varies person to person. You could be an expert in karate but very weak and unathletic. It’s best to have high attributes across the board. Of course skill is one of the most important attributes if not the most important in fighting. It really depends on the fighter and style. Some ufc fighters only fight a few years before becoming champion, they aren’t always 10 plus years of experience. Natural talent of course comes into play. But if they’re the exact same weight and athletic ability more experienced will often win.